I've used Thames Plumbers a couple of times recently and they did a good job and aren't too expensive. The number is 07909234784. They have a freephone number as well but I don't have it with me at the moment.
Don't know the nature of the work you require but can suggest you call Steve Bryant, Bryant Plumbing and Heating (07734051303)based in SE26. No connection other than as a customer. He's also been mentioned several times on an earlier thread.
I'd recommend a plumber called Adam McErchern... he's not with any of the big firms, so should give you a fair price. His number is 07873213110. Friends have had work done by him in the past, and recommended him too.
Oh, how funny, I used this site to find a plumber and employed Paul from Blue Plumbing. Good quote and started the job within two days and finished within time and my bathroom has not failed me since. As somebody new in the area looking not to get ripped off, he stuck to what he quoted, did the job within the stated time and my bathroom (shower/bath/piping/tiling) has been problem free since.