After a couple of weeks driving without the thermostat fitted it has the desired effect of stopping the overheating, unfortunately it worked too well and the engine does not warm up enough to get into the Normal range.
This wasn't doing the engine much good and also the fuel consumption was dropping (engine running rich as ECU thinks it is still cold?). So I have put the thermostat back in. Unfortunately after 100 miles nothing has changed. I need to do a long journey as a real test.