As far as you can see, one career has never inhibited the other, and the 38-year-old Mancunian is enjoying as much success with her second series of The Riff Raff Element as with her three boys and a girl under the age of seven.
Taking a break from both pursuits in a Chiswick cafe, Horsfield reflects on the drawbacks of one of the most demanding double lives since Batman. 'I have a partner (actor Martin Wenner) who's really prepared to go half-and- half . . . but still I couldn't do what Lynda La Plante does - all that research about prostitutes in the East End. If you're feeding a baby every three hours, there's a limit to what you can do. But I'm not complaining. I feel very blessed to be able to do both.'
Viewers have been equally blessed over recent years, with three series of Making Out, the sparky electronics-factory drama, and now two runs of the class-clash comedy, The Riff Raff Element, in which the Tundishes, down-at-heel southern aristocrats, are compelled to live cheek-by-jowl with the vulgar Belchers from Salford. Although no Inspector Morse, the series attracts 3.5m viewers, and the critics love it; the Times called it 'the best thing on television for at least five years'.
Horsfield started writing almost by accident. After graduating from Newcastle University, she and some friends made the pilgrimage to the Edinburgh Festival. 'We decided to write a new play, because if you take a new play you get free publicity in the Fringe brochure. The others were saying, 'let's write about the war in El Salvador or drug addicts in Glasgow' - which made me nervous. I suggested writing about something we knew about, like girl football supporters. Out of sheer desperation, I wrote a play about that in 10 days, and ended up being approached by an agent and three directors.'
She spent two years at the RSC as Trevor Nunn's PA - 'we spent most of the interview talking about football. He's mad on Ipswich Town, so he's not really a football fan.' She won the 1984 Thames Television Award for promising playwrights with True Dare Kiss, which the following year became the first play by a woman to be put on at the National Theatre.
She spent 1985 as writer-in-residence at the Liverpool Everyman. It was here that Horsfield honed her trademark skill: handling a bagful of characters with the deftness of a juggler. In The Riff Raff Element there are 11: if you had to pick the central character, there would be 11 contenders.
She also shows an impressive sleight of hand in shifting imperceptibly between comedy and tragedy. In the midst of laughs, there is death. Laughter and tears mingle in Riff Raff as Carmen, in wedding dress, tries to dig up the body of her boyfriend, whom her father has killed, from beneath the cabbage patch. 'I use the comic to undercut tragic things because that is how the mind deals with tragedy in life,' Horsfield says. 'How many funerals have you been to where at the end people are in hysterics? A marriage breakup can be horrendous, but it can also be hilarious. We have to overcome this correctness, this humourlessness about things.'
Horsfield comes from a working-class family but she now lives in middle-class west London. She has a keen eye and ear for social divisions, and her work contradicts the assertion that we live in a classless society. But her characters are not stamped 'working-class hero' or 'upper-class twit'; they live and breathe - no mean feat in an era when dra-
ma editors seem to commission by numbers.
Horsfield uses the simplest key to unlock her characters. 'It doesn't sound significant at all, but for me it's absolutely crucial to know what someone's called. I'm with Dickens on this one. Never waste a name.' Queenie, Carmen, Boyd, Petula, anyone?
Cue dialogue. Horsfield's, like Alan Bennett's, could have been transcribed from a conversation on the top of a bus, albeit an unusually amusing one. The constipated toff Mortimer Tundish cannot decide whether to shop the low-life Declan Belcher to the police: 'I think for the moment I'm 90 per cent wavering towards not wanting to commit myself.'
The Belchers have an earthier turn of phrase which Horsfield picked up over many years watching her beloved Manchester United (it's not every television writer who shows off a Red Devils key-fob). 'One of the actors in Riff Raff was from Glasgow but was doing a Salford accent. He went to United and said, 'it's great to go to a voice coach, but where you really learn the accent is on the football terrace.' ' And no one can mither about that.
There is little science to Horsfield's art. 'I don't carry a notebook. You absorb things without realising. You meet someone once, and, if you're alert to their speech patterns, a little bell goes off inside. There's an antenna there.'
The antenna will soon be tuned to the Fifties for the fulfilment of a long-held wish: a feature film about the Busby Babes. She is also in the process of adapting a Liza Cody novel about an 18-stone female wrestler ('casting will be interesting'). But for the moment she is taken up with house-hunting in the North. 'I want a season ticket to United. QPR are not quite the same.'
The north-south divide is alive and well and living in Debbie Horsfield.
'The Riff Raff Element' continues on Tuesday (10-10.55pm BBC1).