Can anyone advise on likely causes for non start of the 2.0 HDi

The engine will not start when very cold - fine at all other times

I have replaced all glow plugs with new and it was working fine for several
weeks but then would not start. It has to be said, however, that it was not
that cold when it was working OK

It seems that if the air is very damp it won't start.

I have an electric fan heater which I put under the bonnet for about
20mins - it will start after this.

I don't think that the heater imparts enough heat to the engine to make the
head warm enough to allow the engine to start - could it be that the heater
dries out the air in the intake and this solves the problem?

Yesterday I fitted a new glow plug control box (on advise)

It was OK after this work but this was mid afternoon when the temperature
was more around 12 degrees C.

This morning it was around -2 degrees and the car just wont start.

I could take the car in to have it put on the computer reader. This might
tell me what the problem is but
at 75 quid this seems to me to be a rip off which I cannot afford.

Any suggestions or help gratefully received.

Many thanks

Andrew