Settling the score: Meet the musical brains behind TV's top dramas

If the first chords of a theme tune have you racing for the sofa, these guys have done their job properly. Gerard Gilbert meets the musical brains behind Downton Abbey and Sherlock.

'Classic FM play me more than Mozart," says John Lunn, looking bemused rather than boastful. "It's bizarre… they seem to play Downton Abbey every single day." Countless millions of Downton addicts in more than 100 countries around the globe wouldn't think that there was anything remotely bizarre about the radio station's playlist. But while they will instantly recognise the theme tune to their favourite TV costume drama, with its urgent strings countered by a plaintive solo piano, they are unlikely to have heard of the unassuming, middle-aged Scotsman sitting opposite me. The man who wrote it.

In fact, at awards ceremonies or preview screenings, or on the rare occasions when Lunn has to visit the Downton Abbey set (arranging the Scottish dancing music for a ceilidh in the upcoming Christmas special, for example), even the cast don't recognise him. "Usually I turn up for a screening and the actors have absolutely no idea who I am", he says. "But I know them. I've been staring at them every day for months.

"People think that music is just plastered on to the episode. They don't realise that actually it's very carefully choreographed to every scene and almost every word. I never switch the dialogue off when I'm writing and I always watch the picture."

In fact, apart from Downton Abbey's creator, Julian Fellowes, Lunn has an unparalleled familiarity with the aristocratic Crawley family and their domestic servants. "I know more about Downton Abbey than the directors because the directors come in for a couple of episodes and then they're off," he says. "I've almost done 30 hours of Downton Abbey so I know it inside out. I noticed a tweet the other day where somebody said there was an album of Downton Abbey and they said 'But what is there apart from the titles?' And I thought 'Oh my God, there's so much more than the titles'… "

But then it is the lot of the television composer to be simultaneously noticed and not noticed. "It's all quite subliminal," agrees Lunn. "Whether in 10 years someone will be able to sit down and write a PhD thesis on the musical themes in Downton Abbey, I don't know. At its best people don't really notice it – it becomes part of the fabric."

Some people – the people who dish out television awards – have noticed Lunn. He has repeated Bafta nominations to his name, for his scores for BBC1's Dickens adaptations, Bleak House and (his personal favourite) Little Dorrit, as well as Royal Television Society awards for Going Postal, Criminal Justice and The Amazing Mrs Pritchard. And in October, he flew to New York at accept a Primetime Emmy Award for Downton Abbey. "I didn't expect to win because Americans normally win in that category," he says.

It's all a far cry from Lunn's roots in Steve Reich and Philip Glass-inspired 'systems music', and a time when "Kraftwerk was too mainstream for me." After Glasgow University, he studied computers and music at the Massachusetts Institute of Technology (MIT). Having playing in various free-improvisational groups in London in the early 1980s, Lunn joined an experimental outfit called Man Jumping – once described by Brian Eno as "the most important band in the world". "I've gone from being a radical composer to being more and more mainstream… does that happen to all artists?", he asks with no discernible trace of regret. "I've got two kids and there's no money in contemporary music."

It was composing music for London Contemporary Dance Theatre that would lead Lunn to his eventual vocation. "I met Anthony Minghella… his wife [Carolyn Choa] was a dancer and I ended up writing an opera with Minghella for Channel 4… made a few contacts... and first job with BBC Scotland was The Cormorant with Ralph Fiennes in it."

The Australian composer Barrington Pheloung, best known for his music for Inspector Morse, was also writing for the London Contemporary Dance Theatre at the time, while Michael Price, who co-scores BBC1's Sherlock with David Arnold, also has his roots in modern dance. "You're used to the idea of collaboration," says Price. "It's not just you… it's not just music… "

Price started writing for film and television after meeting Michael Kamen, the prolific London-based American composer who scored the Lethal Weapon and Die Hard films, as well as collaborating with Eric Clapton on the f haunting music to the 1980s BBC political thriller Edge of Darkness. "I helped out on a film score and left five years later," says Price, whose own CV includes the soundtracks for The Inbetweeners Movie and Horrid Henry: the Movie, as well as his Bafta-nominated joint score for Sherlock. Price also acted as music editor (that's to say, he chooses and edits the soundtrack) on Peter Jackson's Lord of the Rings trilogy and Richard Curtis's Love Actually and Bridget Jones: the Edge of Reason.

His title music for Sherlock harks back to the classic theme tunes from the 1960s and early 70s, when the likes of John Dankworth (The Avengers) and John Barry (The Persuaders) created bold and unforgettable calling cards for their TV programmes. "There's a nod to the 60s and 70s in some of the instrumentation", says Price. "Sherlock is a very confident show; from a music point of view, you need to stand up and be counted. I remember a conversation with the creator of EastEnders, who said the music should almost be a call to action so that when you were in the kitchen making a cup of tea you would know it was time to come back and watch.

"I think we might have slightly lost our way with theme tunes. Without being nostalgic I was sat watching the TV the other night and watched The Good Life followed by Dad's Army, and there something very characterful about these individual theme tunes. If you do get something that sticks, it becomes absolutely indivisible from the show."

But how does it actually work? I ask Lunn to talk me through the creative process behind his Downton Abbey theme tune. "There was no title music until episode two", he explains. "[In episode one] I started with the imagery of a train and a lonely figure looking out of the window of a train carriage, and I came up with this solo, single-line piano… quite plaintive… it's actually Bates looking out of the window, he's going down to Downton Abbey and he's not sure of what awaits him… at the same time we're following a telegram… the heir to Downton Abbey has died on the Titanic. And suddenly the music broadens out as the house comes into view. What was the energy of the train became the energy of the servants and when the same music worked for the next scene it became obvious that would be the theme for Downton Abbey.

So there you have it. Whereas Price says that each of the characters in Sherlock has his own theme – counter-intuitively, the military Watson's music is thoughtful and internal ("no drums and horns") and the analytical Holmes is more dynamic ("the music is about the fireworks of his mind and his energy"), Lunn says that he writes specific music for relationships rather than individual characters.

"Having said that, the house has got its own music," he says. "And when I wrote the music for Bates when he arrived in scene one, episode one, it's got this stuttering sort of piano thing – it was subliminal, it was about his damaged leg.

"I can't give too much away about the Christmas special, but one thing I have noticed is that Branson and Sybil (played by Allen Leech and the now departed Jessica Brown-Findlay) have their own music, but since Sybil died in episode five I've only really been able to use that music for Branson when we are thinking about Sybil. But as we progress through the series, Sybil is becoming more and more distant and Tom is becoming more infused into the house, so I've started using the house music – it's almost as if the house at Downton Abbey is claiming him."

Lunn asserts he felt unconstrained by the late Edwardian period in which the drama began. "I listened to a lot of Elgar, Holst and Vaughan Williams… music of the period… it's too overwrought," he says. "Harmony is my big thing, and although the instrumentation is all strings, piano, the cors anglais, French horn… very much 1920s Elgar… the harmony is much more like Philip Glass or Michael Nyman. The harmony is still classical but it's elongated. The metre is not unlike pop music."

Indeed, his music for Downton Abbey has been rearranged and released as a pop CD, Don Black having written a version of the theme tune that is sung by Mary-Jess Leaverland (the British girl who won the Chinese version of The X Factor in 2009). "And there's a big tune between Matthew and Mary at the end of series two, when he proposes to her, and we've turned that into a ballad with Rebecca Ferguson singing it," says Lunn.

Both Price and Lunn compose on computers. This is the age of digital 'sampled music', real musicians recording on real instruments being seen as an unnecessary luxury by the television-drama bean-counters. "There's half the number of recording studios that there used to be", says Price. "At the last count I had 13 Apple computers in my house in Hertfordshire – it's like Jodrell Bank.

"Having the music played on sampled instruments is absolutely personality-free, and we try to make sure there is at least some live instruments on pretty much everything you hear on Sherlock, so that there is at least a layer of something on it. For more emotional moments we try to get that percentage up as high as we can.

"I fight for real musicians… there's no comparison," says Lunn. "The BBC are very good at providing money for real musicians, ITV less so but they have with Downton Abbey… all the musicians are real on Downton Abbey, not a single sample. Sky have still got a long way to go, I think; I don't think they understand the difference."

There are apparently individual favoured musicians, who bring their own distinctive style, such as Caroline Dale, who has played cello on the likes of Truly, Madly, Deeply; Hilary and Jackie and Atonement, and violinist Thomas Bowes, who has mastered the scores for countless films – most recently The Hunger Games. "He was the person leading the orchestra for Sherlock," says Price. "There's also a trumpet player, Derek Watkins, who's played on all the Bond films. I had him playing last year on Horrid Henry: the Movie."

Do Lunn and Price think that the music is an overlooked element of film and TV dramas? After all, while the actors swan around on the red carpet at the Baftas, the music is relegated to the craft awards. "The food is better at the crafts Baftas," jokes Price. "Anyway, for the music to affect you or me, the authorship has to be secondary. That's why we all do concerts on the side to soothe our egos."

There is perhaps a deeper cost to serving the dictates of TV drama, and Lunn says he can no longer write his own music. "The trouble is that I've written so many different styles of music I've no idea what's mine any more, or what's me", he says. "One minute I'm doing Criminal Justice, or Hotel Babylon even, and the next Little Dorrit. My brain's confused. In a way I'm too comfortable.

"With Downton the music has become a part of the branding," he says. "In fact it seems to have become a branding for landed gentry… I notice on the radio when they have anything to do with the House of Lords they play the music for Downton Abbey."

'Downton Abbey' Christmas special is on ITV1 on Christmas Day