I have the MK2 and the power steering sometimes fails when I first try to move away after a car start, Its intermittent and waiting for 1 min and/or revving engine sorts it out for the trip.

Taken it to a garage who said there are no leaks and the is plenty of fluid. I seem to have the correct battery (albeit a Halfords one)

Any ideas please?

Yes, the battery is no good.

You need a cheap voltage/alternator reader and check how it’s behaving at idle