very damp concrete at base of chimney

[pullingmyhairout]

Hope you can help me. I've looked at previous posts but they don't cover my particualr problem - I think.

1930's house. Solid floor and walls. Took out old fireplace down to floor level and on breaking out old cement base found very damp concrete full of large stones directly underneath the chimney. The cement that was covering it was dry on top. This level is below the damp course (bitumen it looks like) but even so I expected this to dry out after being opened up but 2 weeks later it's exactly the same. There are no water pipes on this side of the house and it hasn't rained for 3 weeks.......

Tried to get damp survey but they say they can't do anything about chimneys..!

I thought of putting tanking on it or membrane and then cement but am worried there might be some more sinister reason for it. Any ideas/ suggestions why it should stay damp that long or is it common for aras under the damp course even with no rain...??