Well just after my last post I had a fan heater play on the area, and
surprise surprise, it went bone dry. It was still dry the next
morning. And despite all the rain, is still completely dry five days
later.

So I suspect that it had mainly dried despite the carpet covering, and
the carpet had drawn the moisture up and heating the surface was
enough to get rid of the last of it. Otherwise I am sure we would have
started to see patches of damp again now.

Thanks for all the input.