Hi Booldawg
If your in laws go ahead with a new boiler, DO NOT use British Gas, they are a rip off company. I was quoted in excess of £5000 for a new boiler, installation etc, but if I agreed there and then I could have had an extra discount of £500 - big deal. I eventually had a new boiler installed, power flush to the system, TRV's to all necessary rads, a new radiator in the hall, everything up and running by two superb fitters for £2100. I have never had two more conscientious and clean workers. It's a pity they don't live in Scotland.
Good luck
FBG40