I found LMS Gas Services off the forum and spoke to Mel 020 8699 0665, who runs a friendly and honest Corgi registered plumbing outfit from SE23 who were happy to come up to SE1 and, in my case, sort out my boiler programmer which needed rewiring.
I thoroughly recommend them. Their prices are keen and they have a good attitude.
Just had these guys out to service my boiler and sort out a intermitant fault and they were brilliant so I too recommend them. Very friendly and helpful guy and the bill only came to approx £75 not bad as he was here for more than 2 hours and that included paying the congestion charge.