Help Please!: Heater not working on a 190

Hi - I have a had this 190e for 5 years now - when I first got it, the heater didn't work (fan was ok, just no heat). I left it with the Merc specialist to fix. They said that the vacuum tubes on the controls were loose - presumably they meant the rotary controls on the dashboard? The book (Haynes manual) says that you have to take the whole facia off to fix this - I'm sure they didn't do this (absolutely no sign of it having been removed) - anyone know how they got at these tubes? (reason is that there is no heat coming through again!). Or might it be another problem? I think the heater matrix is ok, because you can get a small amount of heat from it. any suggestions gratefully received! Cheers Andy Bristol - United Kingdom