Following advice from the Club Triumph forum I returned to PMW today.
I adjusted the Revotec fan controller down to minimum which meant that it would cut in at 70C. Here's what the temp gauge looked like then. Higher than would be expected which implies the gauge is over-reading.
I checked to make sure the fan was blowing the correect way. It's mounted in front of the raadiator and I put some paper/card in front of it and it was nearly pulled into the fan so that's OK.
The temperature gauge stayed at this level all the time I went for a short run with the car but I didn't take it too far as it was struggling to get up a small rise in the road in second gear! The timing appears to be miles out using a timing gun but when I tried to adjust it to something closer to where it should be the engine died.
Oh, and the overdrive switch on the gearknob came off again.
More things to ponder over.