We have a two bed in nearby Tufnell Park / Holloway and have used a very reliable plumber since 2001. His name is Andy / Atlantic Plumbing: 07973 182 669 / 0208 361 3613 In the past, he has fitted two bathrooms for us, two new boilers and other bits and bobs, as well as doing landlord gas certificates on our buy-to-let. Very reliable, organised and is reasonable, but not cheap.
If it's more of a builder you're looking for. We can recommend Joe Hayes: 07956979772 Joe got rid of our downstairs loo to make a bigger kitchen, fitted our IKEA kitchen, and turned our upstairs landing into a study area, as well as re-shaping the hallway. Again, he's very organised, reasonable but not dirt cheap. We just loved the way Joe took charge and found him very reliable. However, a friend of ours phoned him for a small job and he didn;t turn up!
Thanks so much for your suggestions. I've contacted Joe and he does seem very competent. I agree not dirt cheap, but he seems to know what he's doing. He has plumbers he uses already, I think. I'll also give london profiles a whirl.