I suppose I need a new battery but the problem seems strange. The car
usually starts fine - but if left standing for a couple of days it
will sometimes behave as though the battery is almost flat.

If the battery is older than five years, bin it anyway before investigating
further. If it isn't dead, it soon will be, so you won't be wasting your
cash. I don't think I have ever owned a car where I haven't had to change
the battery.

Christian.

P.S. Your symptoms seem like standard knackered battery syndrome.