Well after the FIFTH time of removing the thermostat housing i've figured out what's going on!! It's all to do with the £3. plastic/rubber gasket.
Rather than fitting the gasket on the thermostat (as in my picture) the gasket needs to be forced into the groove inside the housing. This took a fair bit of brute force especially as my hands were absolutely freezing!
(I originally thought the gasket fitted on the thermostat as this is how it came out and the Renault manual in it's suitably vague normality states refitting is opposite of removal!)
I also checked the new thermostat in a bowl of hot water with a thermometer to ensure it was opening and closing around 82.5 degrees.
So with the gasket located in the housing, I greased the thermostat shaft with Molykote 111 so it can slide through the gasket and fitted it shaft side into the housing, bolted everything back up, degassed the coolant and it now warms up like it should!! and no deviation from half way when it reaches there either
The problems warming up usually start after i've been on a trackday or it's been really hot so I think what happens is that the thermostat expands and then when it retracts because the core drops below 82 degrees it drags the gasket out of the housing which results in a gap where coolant can flow through from the radiator from cold.
Hopefully the Molykote will stop this happening in the future, as when i've taken out the old gaskets they don't slide very freely over the thermostat.
I'm pretty much an expert at removing the housing now, so if anyone needs any advice let me know!
