What is wheyrock? I was thinking about 18mm chipboard (I have some packs
from our old place which we didn't use). A quick google says its a
composite floor decking - but I've never seen it or know what its like.
Is
it much lighter/stronger than chipboard? What's the sort of cost of it?

sorry, should have been clear - Wheyrock is a trade name like Thermos for
vacuum flasks ... I refer to 18mm waterproof T&G flooring grade chipboard.

Rick