You could try GigCB though they veer towards Breton :) but hey they're local! Contact is Chris Shaw, 0794 026 9223 If they're not suitable I'm sure he'll know of other people. Alternatively, Two For Joy, an offshoot of The Magpie's Nest, runs lots of ceilidhs/barn dances, mainly in North London but they've just extended them into South London, I'm sure they'd know of suitable bands for you. Contact them via their website http://www.themagpiesnest.co.uk I think the guy who organises the dances is Joe Buirski, but my memory's crap so I may be wrong :)) If not, Sam Lee can put you in touch with the relevant people.