Francesca Martinez's victim is squirming. Trapped under the scrutiny of the comedian and fellow members of the audience at her show in Edinburgh, he is clearly wishing for the proverbial hole to open up. "What are you bad at?" asks Martinez. "Football," comes the sheepish reply. "Were you born like that?" she enquires, head tilted in sympathy, "Couldn't your mum have had a test when she was pregnant?" Turning to the man's girlfriend, she simpers: "You are so brave. Well done... Does it mean he can't have sex?"
Martinez's humour bears a political sting. As one of a tiny number of disabled performers who have made it into the mainstream, she is not about to waste opportunities to ram home a message. Born with cerebral palsy, the 31-year-old refuses to accept the label of her condition, preferring to describe herself as "wobbly".
She was 14 when she got her first acting part playing Rachel Burns in Grange Hill, making her the first disabled person to get a major part in a children's television series. Most recently she appeared as the actress Kate Winslet mistook for a drunk on the BBC sitcom Extras. Her no-holds-barred approach to disability has seen her celebrated on the comedy circuit for her biting brand of stand-up.
Sitting in a pub round the corner from her family home in Kensal Green, north-west London, she pauses only once in a two-hour stream of conversation to take a single sip of juice through a straw.
There are few topics that don't prompt a lengthy and passionate response. From religion (she's a staunch atheist) to the politics of Tibetan rights (she caused a storm last year as the first torch-bearer to pull out of the Olympic relay because of China's record there), she is not afraid of taking a stand.
On Thursday she will attempt to raise laughs for another political cause, as she joins a line-up of comedians at the Friends of the Earth LIVEstock gala at the Apollo in London. Coming from a family of vegetarians, animal rights are a big part of her upbringing.
"My mum was a volunteer for Friends of the Earth when I was growing up so environmentalism has always been a part of my life. Both my parents were always vegetarians but they never imposed it on me. I was about 10 when I saw a documentary on battery hens and I started crying. I went down to them and said OK, I want to be a vegetarian. Something like that is always much more genuine if it comes from you."
Understandably, then, issues inform a great deal of Martinez's act. Having been handed a platform, she feels an almost zealous duty to hammer home the politics with her humour. "I know there is a huge variety of comedy out there," she says. "But the way I write jokes is, I think about what point I want to make and then I write a joke expressing that point. I think it's a real privilege to have an audience's time and I feel I should use that carefully. I love being funny and all my life I've loved making people laugh, but there's a deeper satisfaction when the laugh is also infused with something the audience can take home with them after."
Put like this, one could be forgiven for fearing that her comedy is about as rib-tickling as a lecture on Marxist theory. But even when tackling the most serious topics it is rare for her not to have the audience shaking with hilarity. Even the subject of aborting unwanted children is adroitly transformed into comedy fodder. "No one wants an unhealthy baby, but I can imagine being devastated if I was told 'I'm really sorry, your baby boy will be a Tory politician'."
If you still haven't heard of Francesca Martinez, you can blame TV's commissioning editors. Despite several sell-out UK tours; a sackful of silverware and a growing band of enthusiastic fans, she has never been allowed to perform her stand-up material on television. And she is not best pleased about it.
In June this year, Martinez was one of the leading names brought out to promote the BBC's launch of a disabled actors and performers directory. The gesture was a good one, but has done little to salve her irritation at the broadcaster. "The BBC, even though they're a public broadcaster, still hardly ever represent disability on TV," she blazes. "I've been going 10 and a half years and I haven't done any panel shows. I've tried all of them – Mock the Week, Have I Got News for You – but my agent gets told 'we're worried she'll make the audience too nervous'. Ultimately, I'm not saying I should be on TV because I'm wobbly. I should be on TV because I've proved that I'm funny and I can be entertaining. Maybe the truth is that disability is the last remaining taboo. People are so nervous about it."
Martinez's humour punctures that nervousness. As she is helped on to the stage her first joke is often poking fun at the irony that her cerebral palsy means her stand-up invariably takes place sitting down. While most of her shows have received enthusiastic reviews, some critics have sneered that disability is consistently central to her work – and nothing makes her angrier.
"I get asked a lot why I talk so much about the issue of disability and I always say I'm just a woman who talks about her life," she explains with exasperation. "It's not about issues. No one says 'oh, that Jimmy Carr, he's always talking about being middle class and male' – or 'Johnny Vegas, he's always talking about liking cars and drinking'. But because I'm different from the norm, if I talk about being different, suddenly I'm talking about the issue of disability. I never talk about any other kind of disability. I just talk about my life and my views, some of which happen to contain my experience with CP. I'm not doing anything different from anyone else."
For the past four years she has been working on a sitcom for the BBC which she hopes will bring some of these experiences into people's living rooms. Writing in partnership with her father, the playwright Alex Martinez, and the Extras producer Charlie Hanson, she has shot a pilot for a sitcom which she has dubbed "Sex and the City with a wobbly girl in it".
Her whole family has backed her in one way or another: she relies on them to accompany her when her shows tour. She still lives with her parents and brother in the house she grew up in, and now her boyfriend, the actor Kevin Hely, has also moved in.
"There's a deeper bond that's created when you physically rely on the people around you. When I travel I need a companion so that means that my brother Raoul, my mum, my dad, they come round the world with me, whereas if I were able-bodied they wouldn't have. I think that's added a closeness there."
It was her brother Raoul – whom she describes as her best friend – who persuaded her that Tibet was something worth taking a stand over ahead of her appearance as an Olympic torch-bearer. He pricked her conscience, but it wasn't until she was in the taxi to the Channel 4 newsroom that she finally decided to pull out.
"Raoul had been reading a lot about Tibet at the time and said, 'Are you aware of this?' I had this dilemma, because I thought, it's really good that I can represent difference on a world stage. But when I was in the cab going into Channel 4 I thought about what Raoul said about standing up for what you believe in, and thought if you want people to take you seriously ever again, you've got to live by what you say."
She credits her family's care for her with the confidence she has today. "You hear a lot of parents with disabled children say there was a period of mourning and grieving, but when I asked them, they said, 'No. You were just Francesca, all our lives.' Growing up with that had a huge effect."
Francesca Martinez will be performing at LIVEstock at Hammersmith Apollo on Thursday
1978 Born in London to a Spanish father and half-Swedish, half-English mother
1980 Diagnosed with cerebral palsy
1994-1998 Appears in 55 episodes of Grange Hill as Rachel Burns, the series' first disabled character
2000 Wins The Daily Telegraph Open Mic award
2002 Performs debut solo show, I'm Perfect, at Edinburgh festival to sell-out audiences before taking it on a UK tour
2005 Appears in Bafta-winning Extras episode alongside Kate Winslet
2008 Made global headlines after pulling out of being an Olympic torch-bearer because of China's human rights record in Tibet
2009 Hosts the launch of the BBC's Disabled Actors and Performers Directory. Makes pilot of new sitcom with Charlie Hanson, the producer of Extras. Appears in LIVEstock