Technical Request - 1998 A6 1.8T Overheating ? Help!

My temperature warning light came on today. Pulled over immediately. Temp was at the second last notch on the gauge (UK).
Useful facts : No water loss.
Oil in engine. Fan working. Engine does not smell hot.
Other symptoms: The heater temp was inconsistent with the engine temp. i.e. on a high level it was sometimes blowing warm rather than hot. It was hot initially when I put the blower on.
My best uneducated guess is that the symptoms seem to point to a thermostat problem.
If anyone has any ideas on what this may be it would be good to hear from you ?
Thanks
Stuart
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My A6 overheated due to a cracked impeller in the water pump. You don't mention mileage but mine went at 90,000 miles.
If you drove the car with high revs the crack was sufficient to cause the impeller to slip on the shaft and then the car would overheat. Drive it with low revs and the impeller wouldn't slip on the shaft and didn't overheat. Had same heater problems, prbably as the water is not being circulated correctly. Hope it helps.
Ian.

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Thanks for your points though, it could be more complicated than first thought. 2 Mths left on my RAC warranty, I better get to work fast. Stuart
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Ian, I can now confirm that the fault was traced to the waterpump. I hope I caught the overheat in time. Thanks Stuart
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