I can highly recommend Koruso! The Southwark Community Choir
I started this almost four years ago and it's still growing strong. Around 50 members meet Thursday evenings at Oliver Goldsmiths Primary School on Peckham Road from 6.30pm to 8.30pm.
It's not SE1-based but is a borough-wide project - with members and performances across Southwark. They've just had their spring concert and are now working towards a summer concert on July 14 and will be hosting the first leg of an exchange visit with a German choir from Langenhagen (twinned with Southwark) in February 2013. The German choir is called Choir Under Fire and both choirs will perform at an SE1 venue (TBC) on Saturday 16 February 2013.
There's lots more info on the website: www.koruso.org.uk
You'll see there that they have a free taster session on Thursday 19 April from 6.30pm, but you can go along any Thursday evening and 'sound' them out. Just tell them I sent you. I'm not involved as regularly as I used to be but I continue to support them in any way I can and I hope to make a return to the tenor section soon.
Thank you so much! They sound fab, but Balham is just that step too far for me to make it there after work in time. I've enrolled in the Morley College 'Can't Sing Choir' class which is also on a Tuesday evening, but much closer to home. Fingers crossed!