Central heating fitter fitted wrong pipe?

I have had my central heating/combi boiler for over 2 years. Recently I have experienced a problem with the water heating up, then going cold, then going hot again. I have a sort of home cover with homeserve, they have been out on 3 occasions about this in a space of a week. I have not noticed the problem before as i have had the central heating on for 5 months (controlled by indiv thermostats on each rad). Homeserve and a diff gas central heating plumber have both said that the man who fitted it, has fitted a 15mm pipe from gas meter to boiler when it should be 20mm. They said that at first it would have been fine, but now with age, wear and corrosion its not sufficient enough to provide enough pressure. can I report the original gas central heating fitter for doing this? as I now have to pay out for it to replaced. I have tried contacting the orig man but i am very doubtful he will turn up.