Yours was a common fault at the time with the 221 it didn't apply to the others as they didn't have the cooling jacket, ur fault wa usually always caused by burner pressure creeping up, they always did this if not set on every service, in fact due to a number of incidents corgi enforced Vaillant to do a recall on the open fle models to fit a restrictor so the gas pressure couldn't go above recommended levels .this didn't apply to ur balanced flue model hence overheating causing cooling jacket to crack, an if urs melted the top it must have been way over gassed!
