thanks Nick, yes that what I'm thinking now too
Well I decided to put a blanking sleeve in today and use an accurate digital thermometer and the car only gets to 80-85c when standing still, but the Mk1 gauge, using the red sender unit for 64 to 69 cars, shows it still at Hot. I don't think any of those fixes apply to Mk1, so I am going to try and adjust the gauge, as per one of the earlier replies.
should be just the same with a suitable thermostat.