Thanks so much for this - you're right that it does sound like it could be the thermostat more than anything. I changed the coolant temperature sensor originally because it was throwing a code and giving me a check engine light, so it probably did need changing (if only to get it through the MoT), but it's always never come up to temperature fully since, so changing it might have just made it more obvious that the thermostat is failing - especially with it getting worse over time. It's the original, so it makes sense.
Luckily, thermostats are cheap, so I'll get one and a new housing and see how it goes. Found this
video on
YouTube and while I've got the ARL engine, it looks pretty similar.
Now I just have to have a think on whether there are any 'while I'm in there' jobs I could do while the coolant is drained - like if I want to put an OEM temperature sensor in anyway just to be safe. I've always got on with Febi, but OEM is OEM after all...
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