Overheating?

Worcester 35 CDi boiler 10 years old. I have turned the heating control down to almost off and all the radiators are too hot to touch. I used to have it half-way round and that was fine. As a result the bill this year for the same time is almost 3x more which is the fact I am burning gas at a terrific rate. Brirish Gasman been and sees no problem--Said house insulation the problem but that has been improved in the last year .