Hi Alan,
No, the car starts fine and tbh, it hasn't happened again.
My guess IS the early signs of a slipping alternator belt..... I draw this conclusion as when the Cambelt was done at time of purchase, despite my requesting that the associated belts and water pump be done, my best guess is that they weren't (it was bought from a VW dealership, after all!

)
Does this sound plausible? It'll give me something to do over the next few weeks !
Dan