A Community Network for Bowes Park and Bounds Green
Hi Amanda, I was recommended BBS plumbing, who are very local. Their contact is Bernard at firstname.lastname@example.org, 020 8888 3686/079 5604 1148. Not the cheapest but good, reliable and knowledgeable, and Gas Safe Registered. Bernard had to do a boiler repair for me, so I guess you could ask how much it would cost to repair a leak. Hope that helps.
I know this is an old post, but had BBS guys do a great job at converting an antique heating system to a new combi boiler. No issues, drama or hassle. Detailed quote helped me work out what we were getting.