Get that in writing from COVERHEAT and then produce it to potterton. If they say its not a design fault, then get that in writing and put it to COVERHEAT. It then becomes an argument between them 2 and leaves you to sit back and wait for some compensation hopefully
Alternatively, get yourself a new boiler fitted and be rest assured of no problems for the next 10 years. Boilers have come on a lot since then