I'd go Primrose Hill, The Princess of Wales, The Engineer, The Queens or The Lansdowne pub. Nicer than Camden and only a short walk to the Roundhouse. Although a long time since I have been in any of them it must be said.
Bugger all any good near the Roundhouse, so would avoid getting off Tube at Chalk Farm (assuming you're on tube), and instead get off at Camden Town and walk up - loads of places down that end
Thanks for all the suggestions. As it turned out, [MENTION=27447]Goldstone1976[/MENTION] made it out to play earlier than expected, and had already found Cafe Chula (http://www.cafechula.co.uk) with Mrs G1976.
Decent spot - nice margaritas, good tacos and a guitarist duet who played well.