Same rule as applies to insurance - shop around for prices, although I doubt you'll get it significantly cheaper. Anything to do with boilers / central heating / gas seems to be stupidly priced.
Do, however, speak to people to see who they recommend; I lived in a council flat years ago, and the council commissioned a contractor to install new central heating and the installation was a f*** up from beginning to end (plumber drilled through a live cable, radiator leaked, system was over-filled with water, there was no instructions / user guides left for tenants, and to top it off the bloody things failed its first gas safety test at 10 months old {new system ventilated by old flues}).