Thanks to the people who correctly identified the electrical problem on my SIII as a bad earth. If your're interested here are the symptoms and my fix.
Leading up to the electrical failier, the starter motor turned slowly; we suspected a bad battery. There was always a feint smell of something hot after starting. One morning the starter motor turned a couple of times then stopped. With the ignition on, I was getting the usual charging and oil pressure lights. However trying to use _any_ electrical component killed the entire vehicle electrics - nothing worked, no lights on anywhere. After a few seconds it seemed to recover to it's nornal ignition on state.
I first replaced the chassis to engine earth strap. All the vehicle electrics worked and the starter motor turned. However it still laboured, and I noticed a single clip on the air filter was glowing red hot as it turned over. I guess between the battery-to-chassis connection and the new chassis-to-engine connection it was encountering very high resisitance. Somewhere round the air filter it found an alternative route to earth. Next I added an extra earth strap from the main battery-to-chassis point to a handy plate securing the oil filler tube to the side of the block, it even has a pre-drilled bolt hole. With the extra earth it is perfect. The starter turns faster and with more effect than I had ever drempt it could, and the glow plugs heat the combustion chambers in record time. The old SIII is now very easy to start (we recently fitted Paddocks parallele Glowstart plugs, these are a vast improvement on the old coil plugs but might have caused the earth failier?) Hope this is of help to anyone with similar problems to me.
Thanks again for the advice. David