What should I use to finish on top of a damp concrete subfloor where it is known that there is an underground spring & well?

I assume by your question that your existing floor is water tight and you only want to increase the head room. By the sound of your description of the floor construction, it sounds like this was done for a reason, and if you have no damp, I would leave alone, just for the sake of a couple of inches. If the floor was well tanked, you may have broken this by digging your trial hole. As for a 4 inch fall, either done by some one who hasnt got their levels correct, or done for a reason, ie falling to maybe a sump pit with auto water pump. If your intent on reducing the levels it could be costly.

With out seeing the floor its hard to say exactly but I would say the answer from B J D Building is a fair assumption and I really would consider leaving it be as it could end up very expensive. richard A&R Ground works

Either leave thing alone as advised. Or be prepared to pay for a full membrane system which would include a sump & pump. You can contact Sovereign Chemicals or Triton in order to get one of their approved local contractors to give you a price. The manufacturer's rep will visit the site with the contractor to draw up a specification for the job. The fact that there is a spring means that a tanking system will not suffuce as there is the possibility of very high hydrostatic pressure from below. Soveriegn will not guarantee a tanking systen in this instance and will only reccommend a mebrane system. No corners can be cut or every penny spent is potentially wasted. For this size basement you will be looking at £6,000 to £10,000. It needs a mambrane wall to floor, an integrated drainage system needs installing to take water to the sump/pump, the sub-floor needs to be checked for structural integrity, the membrane (on the floor) will need a screed on top. It's not a small job. Leave it alone unless you really, really need the space and will see a return on the investment. Good luck

I suggest that the top layer is removed , you then build a support frame using tanalised timber batons (this will allow you to level area, then install a moisture resistant board top (although moisture resistant we advise the underside of the boards are treated prior to installation) leave a 10 - 18mm gap around edge . once floor is installed you can paint with an anti slip rubber type paint. Hope this helps . sales@dampuk.net

You need to keep the spring water flowing through Giving it a good exit ..french drains ? further investigation is needed Resolve the situation as damp water flow will cause problems elswhere. After sorting out water .Part Tanking system can be fitted Polycarb.Type Board over Good Luck