Hi Bruce, thanks for your comprehensive reply.

I have no reason to think the builder is incompetent, in fact as
builders go I think he is very competent. I also do not think there
was actually a DPC under the existing (very old) concrete floor. If
rising damp was the problem, then we would be seeing signs of dampness
on the existing floor by now, I am sure. So I am reasonably confident
that we only need to wait until the 100mm of concrete dries (which it
has not been allowed to do), which is why I am trying to get an answer
to this question specifically. If and when it does not appear to be
drying properly, then we will have to look at what he has done.