We've just finished re-erecting a WWII Anderson Shelter in the garden

Why?

We found it in a neighbour's hedge and it seemed a shame not to. We have
a smallholding and very little about it is modern - an Anderson shelter
fits in rather well, and I don't know anyone else who has got one ("as
new" anyway) in their garden. It's a frost free place to store spuds and
I have a yen to grow musshrooms in it too...

Ah, I think you mean one of the ones made from "wiggly tin" don't you?

Crinkly tin it is, only much, much heavier than corrugated iron. Any
more planting thoughts anyone? Which vegetables benefit most from very
freely draining conditions?