i would just argue the case that there wasnt a contamination before he had messed with the boiler so he is obviouslt at fault, also i would mention to them that any gas engineer that needs to change 9 parts before he gets the boiler to work needs to be sacked as he obviously has no idea of how to diagnose a fault and has just changed every part until he has go it to work. if the diagnosis was correct in the first place only one part would have been changed and the total cost of the repairs would have been nowhere near what they costed at, but that's Vokera for you.
NHS GASMAN, March 2011