A blocked heater matrix can be difficult to clean out, and there's a fair bit of work removing it from the car. Is the front heater working properly? If it is, the heater matrix is not blocked - and there's two heater matrixes on the Galaxy anyway, so the chances of them both being blocked are slim. In any case, any blockages in the cooling system would tend to force engine temperature up, not down. I would insist that he changes the thermostat and rubber seal. Deterioration of the thermostat seal is not uncommon, and can cause the same symptoms as a stuck open thermostat. Any fault with the thermostat or seal is often obvious once the old one is removed and inspected. I find it odd that he's so reluctant to change the thermostat, it only takes a few minutes and the insert type fitted to the 2 litre petrol is only a few quid - It makes sense to change it regardless, if only to rule it out as a cause of the problems. heaters warm up with car and produce warm heat before the temp gauge moves,after about half hour the temp gauge will read at 12 oclock when ticking over,heaters still blowing hot air as soon as i drive the temp gauge will slowly go down into the white and after about 5 to 10 mins driving the heaters start to blow cold air,where is the thermostat,is it located uder the coil pack on the right side or is it on the left under the altarnator