Pilot light goes out?

My mother-in-law has a Main BF water heater which is nearly 2 years old. When boiler was 10 months old the gas installer replaced the thermocouple because the pilot was going out. As this did not cure the problem I contacted the manufacturer who sent an engineer. He said as the boiler worked for a few days and the pilot did no go out whilst he was there he said it may be the over heat stat. The boiler worked for a few more months and the problem returned. I spoke to technical at Main and they advised it could be the overheat stat which was then replaced. After 2 days the pilot is going out again. I have noticed that if the hot water tap is turned on to allow the main burner to light and then turned off, the pilot stays on but if the main burner is left on to fill a bowel for washing up the pilot light will not be alight after the tap is turned off. Logic suggests the system IS overheating. We want to ask British Gas to provide a maint' contract but need to make the boiler funtional first. I have given all details in the hope someone has seen this before.