Thanks Craig for the link, and also to Il Dottore and Catfunt for the original thread. I have just re-soldered the joints on the PCB as it says in the thread and hey presto it works! Took about 2 hrs and fairly straitforward but you need a soldering iron with a very fine tip and a desk top magnifier helps if like me you are getting a bit grey haired and your near sight isn't what it was! Just thought I would add to the excellent instructions, for the benefit of others, the procedure I used to dismantle the control unit once I had got it out of the car.
1 Remove chrome plated rings from front of unit by carefully prising them off.
2 Remove what I think is the temp sensing unit from the back of the panel. This is the large black round plastic moulding. I had a bit of trouble at first but eventually a slight twist anti-clockwise, wiggle side to side, and pull, and it comes out.
3 Remove six small screws from back of panel. I think these are what you call "Torx" screws i.e star shaped socket in head. They are very small but I managed to get them out with a small flat blade ("electrical") screwdriver.
4 Release the two plastic catches on the casing on the opposite edge to the screws, and remove it to reveal the circuit boards.
5 Assembly is the reverse of the above.
Cheers and thanks again for saving me a lot of money on a replacement panel.