Why did my gas fitter say I needed no room thermostat?

I have always had a room thermostat until I had a new combi boiler recently. As I like to keep my house at a temp of at least 12 to 15 degrees at night, I feel I need a stat. How easy is it to have one fitted? My calls to the fitter have been ignored so far and I feel that as he has had the big money from me, that he just doesnt want to be bothered with the small stuff.