I asked the same question recently and got these replies, haven't used anyone yet but thought I'd pass them on for you too
http://www.assureps.co.uk/ - these guys have been great for me x
www.assureps.co.uk
Try John Hall at SynergyHeatsource. They recently installed a new boiler for us and were very helpful in assisting us with choosing which model to go for.
pimlico plumbers. They are fab. Ask for jeff brown. He is lovely and, for interest, his wife is a midwife.
Happy dog plumbing 02086723663 local business run by tooting dad. Have done lots of work for people on these groups and streetlife. They offer free quote/visit for new boilers think