Steve..your spongy floor is caused by delamination you need to get a floor injection kit (about 40 quid) and a big suringe ,drill a series of holes the same size as the end of the suringe in the centre of the delam then drill some smaller holes about 2mm around the outside of the delam area have some wooden dowels ready to plug the holes as the injection fluid ouzes out.You'll also need to tape up any joints and entry points in the floor otherwise the fluid all falls straight through on to the driveway.Allow a couple of days for the fluid to harden then refit any carpets and furniture you have removed to gain access to the delamed area.