In reply to leon.rothera:
NLMC - North London Mountaineering Club might be a good start. They meet at Mile End on a Wednesday evening and the Castle on another evening but at the moment, there's a good bunch of regular climbers who leave London every weekend it's dry.
Don't let the Mountaineering tag fool you - we're a mixed bunch of boulderers, sport and trad climbers plus a few adventurer seekers.
http://www.nlmc.co.uk/
Post edited at 16:28