Well I received new water pump & thermostat on Monday morning, so replaced
them on Monday night.
I'm still convinced that it was the water pump that was the problem, but
could see no obvious faults with either.
Anyway, fingers crossed, it's been OK since. But after changing the fan
clutch & flushing it was OK for a week or so.
Will keep an eye on it, and post back here if problem re-occurs.