Sex, lies and Parliament: What happens when the woman is on top in a power couple?

The stars and writer of a new BBC drama 'The Politician's Husband' tell Gerard Gilbert.

The eyes have it: David Tennant and Emily Watson in ‘The Politician’s Husband’ (BBC)

Since we've recently been dragging up the political past – who recalls Prime Minister John Major's "Back to Basics" campaign?

In classic style, the family-values crusade came unstuck when a succession of Conservative MPs were exposed as adulterers, and this was the starting point for Paula Milne's Bafta-winning 1995 BBC drama series The Politician's Wife. Juliet Stevenson was heart-wrenchingly brilliant as the no-longer-so-dutiful MP's spouse, Flora, while Trevor Eve played the sleazy husband.

After an 18-year gap, Milne has returned with a sort of sequel, The Politician's Husband, only now it features a parliamentary power couple, Aiden and Freya Hoynes, played by David Tennant and Emily Watson. Both characters are MPs this time, Aiden the cabinet high-flyer on the cusp of heading a leadership coup when we first meet him, and it's another rich Paula Milne stew of power, sex, betrayal and revenge. But what took her so long to return to the subject of arguably her greatest hit?

"Immediately after The Politician's Wife people wanted me to do a follow-up", she says. "But I had a very strong instinct to leave well alone. You don't screw up what was good and turn it into a piece of merchandising. But if the narrative engine of The Politician's Wife was to look at politics through the prism of a marriage, then it seemed to me that it could be done again."

No political party is specified this time, Milne wanting to suggest that the machinations shown in The Politicians Husband are not specific to any one side. Also, the drama is about the governing party, "and then it would become about Coalition politics", she says, "and I wanted to get at something else really, which is people's disenchantment with politicians."

But hasn't that disenchantment already found acutely hilarious expression in Armando Iannucci's The Thick of it? "The Thick of It was great because it made disenchantment feel OK, instead of people just feeling pissed off and angry", says Milne. "Obviously a straight drama has a different role and that's why you need this emotional engine to get into it. You need to see a marriage and a family.

"That's the heart of it… the power within a marriage passing from a man to the woman", she says. "I mean, I've been married and divorced twice and what part my success played… it was there… it was part of it… being the more successful. It is very difficult to run a home when two people are in highly demanding jobs. Usually somebody has to give."

Emily Watson would agree. The 46-year-old actress, who won a Bafta last year for her part in ITV's Fred West drama Appropriate Adult, gives another unostentatiously powerful performance as Freya – used to playing second fiddle to her husband's career, but suddenly seeing a way to fulfil her own political ambitions.

Watson has described herself before as the "breadwinner" in her marriage to fellow actor (turned writer) Jack Waters, and recently her husband has been looking after their children Juliet, seven, and four-year-old Dylan, while Watson has been in Berlin to film an adaptation of Markus Zusak's novel The Book Thief. "I'm commuting home as often as I can", she says, "and so Jack's obviously holding the fort, but the boot will be on the other foot once I've done. It's a very fluid thing in our industry."

The most high-profile political power couple at the moment are Labour's Ed Balls and Yvette Cooper – respectively Shadow Chancellor and Home Secretary – although Watson says she was careful not to base Freya on anyone in particular. "There's an archetype there of the man who would be king and his very powerful wife, and you can put that scenario in any time in history and any political set-up. The personal politics between the two of them is very interesting – they have a sexually edgy relationship."

Indeed, as with the earlier The Politician's Wife, Milne uses the couple's sex life to illustrate the shifting power balance within their relationship – Freya rolling on top of Aiden in the first episode, while, in the second episode there is a shocking role reversal. "Obviously doing all the bed stuff was… you know, 'come on Emily, you're too old for this game now, come on…'," laughs Watson. "On this, because it was so germane to the whole thing, it wasn't just one… it was three days of it. We kind of got the giggles really, like 'what are we doing?' ". She then imitates being quizzed by her children. "'What did you today, Mummy?'; 'Oh, met Doctor Who…'"

Watson says she enjoyed Freya's sharp suits ("female politicians still dress like men"), while Tennant focused on his character's hair. "Looking at political faces on the news, I was struck by how coiffed they are", he says. "They often seem to go for a sort of Eighties soap-star kind of look.

"Filming the drama certainly gave me an insight into why politics appeals to people – particularly that sense of the intoxicating effect of power. It's sort of Shakespearean in that way. It's like a history play in the making."

Tennant says he was unwilling to tap what he calls his "social links" with friendly politicians for research purposes, although he did re-watch Milne's 1995 series. "Paula really caught that moment in time when the ruling classes began to realise that they weren't able to get away with whatever the hell they liked any more."

Has Milne again struck lucky with her timing, what with David Cameron coming under potential leadership pressure from the likes of Boris Johnson and Theresa May, while Margaret Thatcher's death has refreshed memories about her political assassination?

"It's hugely about the zeitgeist and it's really good that the BBC are putting it out fast", she says. "That's what Michael Grade said when he got the scripts for The Politician's Wife. 'Make it and put it out fast'. It's such shifting sands politics, you have to put it out before something devastating happens."

'The Politician's Husband' begins tonight at 9pm on BBC2