the oil temp and fan switch seem to agree the temperature is lower than the coolant gauge says, since you've swpped rad and stat and bled system with no change suggests to me its more down to the clocks or wiring. do you have access to another set of pre-90 clocks? any will do just as long as it has 2 plugs, even mk2 scirtocco ones fit! if you take out the dash under tray you can pull both the plugs off the clocks and swap em to a new set for testing without dismantling the whole dash. If you're anywhere near me I have a set o pre-90 16v clocks we could try.
edit: just had a thought, what colour is the plastic ring on the sensor? it should be black
