We have used Mark at Pure Plumbers to repair things before, on the back of their CheckATrade recommendations. Can't complain about his workmanship at all. I think he charges a flat ?60/hr +vat, nice and simple.
http://www.checkatrade.com/PurePlumbersPart suppliers in London for the Potterton boilers can be very expensive if you do need anything replaced, just as a note of warning.
We have British Gas homecare now to cover is in the long term, it just saves hassle. If you need anything done by British Gas WITHOUT cover, they charge a flat ?200 or so (in this area) to come out, do repair/service/whatever and go. In this area, for pure boiler/control coverage, its ?11/month, with ?50 fixed fee callout (no other charges). Annual service is included with that though iirc, so you get your ?11/month's worth. If anything goes majorly wrong, its a flat ?50 fee (ontop of your ?11/month). Thats called Homecare 100 Flexi - if you think a service will take more than an hour, then british gas is worth a look. I have no idea how long a service should take :E
We have normal homecare (not flexi) so we don't pay callout fees, thats just for my peace of mind at the moment with all the hassle i'm having with Toynbee - if you have a trouble free boiler, then Flexi looks like a good option, as for ?132/year you get an annual service, and relatively good cover for one of the more expensive items in your home to repair at a fixed fee of ?50, which covers all parts+labour should you need repairs.
Tip for anyone getting British Gas homecare cover, use quidco or similar cashback sites, and get upto ?40 back for signing up, that brings your first year cost down to an amazing ?92!