Well just to close this one off, I bit the bullet and changed the thermostat. Mahle TI 15 87 is the same part as VW genuine. It even has the VAG logo on it, but scratched out.

After a slight worry when the overheating continued, the check coolant light came on. Topped up and has been fine since - temp stays between 89C and 94C at all times.

Thermostat job not too bad, I found the bottom thermostat to engine bolt the hardest to remove. Took 4 hrs or so all in.


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