For future reference it did prove to be as I suspected. It shouldn't happen that the heater has an effect on the operetion of the clutch, but it did!

Yes, the master cylinder must have been defective in some way, presumably internal wear on what is after all just plastic. However, replacing the master cylinder has cured the problem, whether the hearter is on or off!!!

well done Ken on your solution, just shows what else can influence the master cylinder,

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