A London traffic snarl-up-from-hell did its best to ruin my fun last night. Closures around Hyde Park meant I missed the first two acts at the annual benefit for engagingly experimental radio station Resonance FM. I've investigated on your behalf and discovered that Robert Newman did a slightly overlong bit about evolution, presumably something from his solo show, reviewed here. By coincidence Rosie Wilby then also did some stuff inspired by Darwin, with particular reference to tall lesbians and short gay men. If you like the sound of that you can see a clip from a previous Wilby show here.

I thought I had missed an appearance from renowned Canadian has-been, or should that be never-was, Baconface. But he was apparently dropped from the bill, not because of lukewarm reviews for his recent Edinburgh shows, but because two extra acts were late additions. More about them later. First up in the middle section was Shazia Mirza, who took a while to come on. Compere/curator Stewart Lee* claimed she was still in her dressing room when he announced her, but Mirza claimed she was in the wings but he hadn't seen her because she was all in black.

I hadn't seen Mirza in a while – she says she has been working in America where people think she is Mexican and had a good undercutting gag about the kind of work she gets there – and enjoyed the first half of her short set a lot. Her material about race is insightful and sharp, particularly a routine about applying for something where she is asked where she is from and she explains that she is born in the UK, only to be asked, "before that". The rest of her set could not quite live up to this brilliant running gag although I really liked the story about the dim American who said to her "You have an English accent, how did you get that?". Mirza is as deadpan as ever, but she has certainly come on big-style since her early days when she was over-promoted in the post-9/11 rush for burqa-wearing comedians on panel shows (she dropped the burqa years ago). This set was exactly 12 years on from 9/11 and she naturally had a very good, suitably dark gag about that too.

After an atypically macho high five from Stewart Lee, first late addition Tony Law was on amiably manic form, explaining how he was just back…from Space and peppering – or padding out, as he put it – his inspired noodling with references to songs by the Who. He also quickly explained how he was delighted to be doing a benefit for a change that wasn't for people who were dying. If Law found his voice in Edinburgh last year he hasn't exactly lost it, he just doesn't know yet what to do next with it. He won over the crowd with a mix of bluster, buffoonery and digs at skinny-jeaned young comedy upstarts, but then didn't quite know where to go from there so cannily left them wanting more. Which was probably a wise move as I'm not sure how much more he had that they would have wanted.

After the second interval Lee returned to introduce the second special guest of the night, Simon Amstell, who quickly quipped that he was taking advantage of not being paid to try out some new material. I love Amstell and it was all really funny if not particularly different from his old material. I won't go into too much detail here as it was a try-out, but he is still tortured by angst, still unable to live in the moment or grab life by the lapels. He recalled missing out on an orgy with a boy band many years ago, then explained how he more recently missed out on an immersive theatre experience in New York. It wasn't clear which missed opportunity he regrets the most. Amstell clearly spends too much time thinking and over-analysing and not enough time doing, but he still seems to be doing OK where comedy is concerned, even though while he was wondering aloud what to do next, one clot in the audience – surely not a refined Resonance FM fan – suggested he walk offstage.

And finally the headliner. Or the "default headliner" as London-based American Lewis Schaffer was dubbed – he'd been doing his own solo gig in Soho so had to come late. Schaffer has a show on Resonance FM so deserved his place on the bill and earnt the top spot with quite possibly the best gig of his life and possibly the biggest audience too. For a change Schaffer seemed to storm it, with a perfectly balanced blend of self-hatred and insulting his audience. For every gag putting down the English crowd the fast-talking neurotic New Yorker had a gag putting himself even further down. Lob in a few low-status Rupert Pupkinesque self-mocking remarks about his childhood, a continuous self-critique and the occasional edgy paedophile reference and this may just have been the accidental anti-comedian finest 15 minutes as he left the stage to near-rapturous applause.

For Schaffer, however, things are never that simple. Amazingly for a benefit the gig had finished early so Lee got Schaffer back on and he promptly managed to dig a hole for himself with an anecdote about being happy to play a venue that wasn't in a basement - cue mandatory Joseph Fritzl gag - which meant that for a change people in wheelchairs could see him. He followed this with a story about a blind fan which I've actually seen before and has a genuine point to it as well as a brilliant pay-off. Rather annoyingly another clot - or maybe the same one - slightly stepped on the punchline, spoiling the moment, but then that sums Schaffer up. If he can't snatch defeat from the jaws of victory himself somebody else will do it for him. 

*Props, as they say, to Stewart Lee, who was on particularly good, relaxed form even though he protested that he was no exuberant Live at the Apollo-style MC. I won't review his links in-depth here as they were basically mini-warm-ups for his next BBC TV series, but they certainly boded well. His thoughts on politics felt a shade more mainstream and crowdpleasingly satirical than his past controversial routines. Maybe his wife has told him to pull his finger out and write some proper jokes...