I was told by a gas engineer that leaving your boiler running was more efficent as the boiler stayed hot therefore burning the gas more efficently. I tried this and used a hell of a lot of fuel more than when it was on a timer and went off. I also had a timer from a company called warmworld, I think, and it cycled the boiler on and off claiming that this saved gas which it obviously will if its off!! So the best thing to do is, try it and keep a record of your fuel used in a 24 hour period, if thats possible.