If it is just the Electrical side of it, and you are comptent at wiring then Honeywell do some pretty good data sheets/diagrams - most plumbers merchants can provide these.
Really depends on the setup - what type of boiler, do you have a hot water tank, how many thermostats, seperate hot water & heating etc etc.
As has been said above, if you need any Gas fitting done, get a CORGI person in.