You have the classic signs of a failed thermostat. Unlike the early opening problem the diesel stats became infamous for, they can also fail to open. It's the way it goes. If the wax capsule is compromised, the very strong valve return spring will ensure it stays shut, no matter how hot it gets.
I'd suggets you read through this a few times:
https://www.the75andztclub.co.uk/for...ght=thermostat
- - get out your tool box out and watch the weather forcast on BBC1. You'll need at least a good day's sunshine to fix it. Remember to buy a new o-ring that seals the metai coolant rail to the stat housing. I believe these are separate parts from the thermostat housing kit and is also available from Rimmer Bros, forum traders and eBay.
Good luck.
TC