The Sessions, BFI London Film Festival

John Hawkes and Helen Hunt in The Sessions

Based on the true story of Mark O'Brien, a journalist from California bed-bound since childhood after a bout of polio, The Sessions is the improbably cheery, touching and surprisingly funny story of Mark's quest to lose his virginity. The Forty Year Old Virgin meets The Diving Bell and the Butterfly, if you will.

It's an unlikely tale. A commission to write a piece about sex and the disabled opens Mark's eyes to the possibilities of having a sexual life after spending most of his 38 years in an iron lung and assuming such things were beyond him.

With the support of his ball-breaking assistant and his reluctantly liberal priest (William H Macy on stirling form), he works up the courage to employ the services of a "sex surrogate". "The difference between me and a prostitute," says the surrogate (played by Helen Hunt) on their first appointment, "is that a prostitute wants your repeat business. I don't". Their limit, she explains, is six sessions. So what will happen between then and now?

We recently saw John Hawkes, who plays Mark, as a violent meth addict in 2010's Winter's Bone and as a cult leader in last year's Martha Marcy May Marlene. So he seems an unlikely fit here and yet he gives an incredibly endearing performance in a role that many would overplay.

Given the physical limitations of the role - he can move nothing but his head - it's also remarkably weighty, stealing the limelight away even from Hunt, although she too deserves praise for bringing her particular brand of kind but sensible to the relationship.

Without the restraint of both leads their scenes could easily have been shmaltzy. In fact most are anything but. Indeed, Cheryl's clinical approach to their liaisons takes some getting used to, for the audience as much as Mark. Director Ben Lewin lightheartedly exploits the kinds of absurdities in sex that everyone, not just the disabled, will be able to relate to.

It's disappointing then, that the film loses pace about two thirds in. The scripting (also by Lewin,  himself a polio survivor) starts to feel manipulative where once it seemed natural, as do the abrupt scene changes. When Cheryl's Judaism teacher tells her "this is the body God gave you" and immediately after we see her repeating this to Mark, it all starts to feel a little forced, as does the sentimental score's crescendo.

There's something a bit lacking in the sum of these parts, rather than the parts themselves. It's clever to tackle such a tricky subject with humour, but where is the pain? The ending feels a little too neat, in a way that reminded me of Juno. It's great to see strong characters resilient in the face of unthinkable hardship.

But is it all really that easy?