Anyone know a heating engineer?

My central heating system is pretty old and the service engineer who fixed a boiler fault a couple of weeks ago tells me that the pipes and probably radiators are clogged with sludge and need to be cleaned out, either chemically or by something called a power flush (which appears to be fairly expensive). Unfortunately my service policy doesn't cover that, so I need to find someone to do it.

Anyone able to recommend anyone in the London area? Preferably cheap and reasonably good?