Damp musty smell! Help please.

[happyclapper]

There is a horrible smell of damp coming from our cellar which has been converted into an office...tanked out etc. Cannot find any damp patches but there is mould growing on some books etc. The hummidty is high so we have a dehumidfier but this is too expensive to have on all the time.
Don't know what damp proofing was done before we moved in 3yrs ago or know how to find out.
Any suggestions gratefully accepted.

Is it properly vented? When we got ours tanked, it smelled awful, and we later discovered they had completely sealed the whole thing so it was just one big, airless tomb which never properly dried out.

Sorry for delayed response. The door that leads down to that room is generally open so air csn flow freely up into the kitchen. I don't know if that would be sufficient. The smell seems to be worse at the top of the stairs but all the walls there are internal. Uhm.....