The dash was stripped to check the heater matrix, there were no signs of current leakage although the garage has said there is evidence of the carpet being taken up before so perhaps historically there was one? I'm thinking if there was a leak in the matrix, this could have caused the head gasket? There was also signs of sealant in the coolant reservoir so perhaps they had tried that the seal the matrix? don't know, there is also a receipt for a new water pump. The filter was changed from the engine side, just to clear up any confusion. Is the pollen filter located behind the coolant reservoir and it;s about a foot long, that's what was changed, I know it was changed as they showed me the old one they took out, the new one is white and clean! I have switched the rear heater to low and cold setting and I still get the smell when I switch the front heater on.