Now that the winter's gone I'd be grateful for any advice as to how to get the heater in my wife's 300TDI hot again..

Basically the heater fan in fine and the heater blows cold with the temperature dials in the 'cold' position. However, moving the dials to the 'hot' position only provides a (bearly perceptable) increase in the temp of the emitted air. Thinking it could be an air lock I've dropped off the heater matrix return pipe this afternoon only to find that no water came out (the matrix water valve feed pipe from the back of the cylinder head was hot).

From this I've diagnosed 3 possible reasons for the cold heater:
1) A blocked heater matrix
2) A stuck heater matrix valve
3) a linkage malfunction between the heater matrix valve and the heater temperature controls

From your experience does this sound right? If so which of the above is the most likely to be right? Finally, how can I get to the back of the heater controls / heater matrix / heater matrix valve to sort out the problem?

Many thanks in advance for your advice!

id say a blockage or not been bleeded properly from my experiance.

I think the whole dash has to come out to get to heater matrix big job
Seem to remember some one else had the same problem.

If no water came out there's a blockage more than likly. Take both pipes off and make a pip to fit the exit side of the matrix and stick your garden hose to the other en of that pipe. Run clean water through it backwards and if its blocked will/should eventualy clear. If it doesn't. It's dash out time.