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So, I have always had a problem starting the car back up again when the engine is up to temperature, and more so on a hot day.

BUT, today as it cranked over and over as usual at the petrol station, for some reason I released the ignition key VERY slowly, and the engine fired up instantly - at some point in the spring return phase of the key movement. Tried it over and over, the slow movement that is, and the car starts instantly ever time. Something it has NEVER done hot or cold.
I therefore am thinking there may be a fault (or dirty contact) in the ignition switch, and that there may be a link to the ignition ballast resistor around this point.
Does anybody know what happens inside the ignition switch at this point in its travel ? Obviously it is not just a case of powering the starter (as that always turns) but something else must happening in addition. Or should I just remove the switch and get going with the old trusty contact cleaner.

Cheers all,

Sean.

When starting the ballast resistor is bypassed and 12v is applied to the coil from the starter relay. Once you release the key the supply to the coil is via the ballast resistor which reduces the coil voltage to 6v.

On the '81 wiring there is a white/yellow wire to the coil. Connect a multimeter between the wire and earth/body. With the ignition key in the off position there should be zero volts on this wire and when you turn the ignition key to the start position it should read 12v (DC).

In the run position it will be difficult to measure, but the white/yellow voltage, when connected and running, should be around 6v.

I suspect that there is a problem with the starter relay circuit & as you release the key there the engine is still turning and there is just enough power flowing through the ballast resistor to generate a spark and start the car. Firstly check all the connections to the starter relay, clean and grease. If this doesn't work, try replacing the starter relay.

Releasing the key also means that the starter motor is switched of, and as such all power goes to the ignition (through the ballast wire).
That should be the starting point of your search, together with Howard's remark

Verdigris

"a bright bluish-green encrustation or patina formed on copper or brass by atmospheric oxidation, consisting of basic copper carbonate"

Or in the form of a jelly like substance in the headlamp switch.

Think I will start with the starter relay then - Haynes tells me white/orange and white/yellow come from that relay.

Thanks for the info guys.