Vaillant TurboMax Combi problems

My Vaillant combi boiler has been playing up for a few weeks now. The first symptom was that often there was no hot water available whilst the heating was on.
But now it's started staying on (boiler firing, pump running) even when the timer has turned off. Not often, but certainly more than once!
I originally thought it was the diverter valve (already replaced once under warrenty, though sadly it's now 3 1/2 years old) but now I'm not sure..!
Any ideas?
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