Hi,

I've got a mk 3, 2.0 tddi, 116,000k on clock 51 plate.

That's the car, now the problem. The temp gauge is normal, engine running well, but now that it's a little cooler outside I turned up the temperature in the cabin. No heat, coolant level is normal and fan works fine. At about 70mph, some heat comes through, only to disappear when I slow down for town driving.

Any ideas, thank you in advance for any advice.

check the hoses in the engine bay, the thermostat may be stuck open meaning that its constantly being cooled. Also worth checking and replacing the heat matrix resistor behind the dash.

it won't be the main thermostat or it would take ages to heat up (if at all)

there is a second, oil cooler thermostat (at the side of the waterpump housing) - but if the car is warming up as normal, and maintaining its temperature (about halfway up the gauge) its unlikely that this is gone, either

Mk3 diesels take a while to warm up, and do not generate an awful lot of heat -because of this there is an electric auxillialy heater inline between the engine and cabin heater matrix, (in artic countries it is diesel powered ) it may be this that has blown or the matrix blocked, an airlock, the heater controls may be faulty or the car may be fine (because thats kinda the way they are)

i take it the AC is switched off and the heat turned right up? (manual AC)