[Peter Hoskin]

Many thanks for all responses, i have learnt a great deal. Further tests of my instrument circuits showed the following:
1) Supply from the ignition switch to the A,B,C,D fuse panel had a faulty connection to the fuse panel,
2) the spring clips retaining the fuses were in some cases rather loose,
3) supplies from C fuse (not A as shown on the wiring diagram). The two green wires supplying the instruments and handbrake warning light were badly burnt.

Replacement/correction appears to have restored everything to normal, the fuel and engine temperature gauges are performing as normal so the voltage stabilizer would appear to be ok.
Where is the stabilizer fitted? Still not known but I recently replaced the instrument lighting with LEDs and saw at the back of the panel steel support frame two metal boxes mounted on the ENGINE Side of the frame at top left and right corner and thinking at the time what a stupid inaccessible place to put whatever they are. One is probably a stabilizer.

I cant be certain what caused the overheating so I have reduced the C fuse to 35 rather than the 50 original and have done about 150 kms without a problem.