I'm not sure what you mean by "capped off in the loft" do you mean that the chimney has been taken down below the roofline and a cap fitted on it inside the loft? Surely a cap is fitted to stop the ingress of rain, which wouldn't happen if it is below the roofline
Anyway, that aside, I had the same set up (except there was no cap, the chimney top was just left open into the loft) and damp, so I installed two air bricks one inside the room at the base where the fire place would have been, and one outside at the top to get a flow of air through the chimney. It reduced the damp slightly in that I suppose it helped dry it out, but didn't get rid of it, so I opened up the chimney breast and found muddy sludge at floor level. It appears that when the house was built (1960's) the chimney base was open to the ground. so I dug down and removed about 2 feet of sopping wet soil, lined the hole with a piece of damp proof membrane and filled the hole with concrete. That was 10 years ago and there has not been a trace of damp since.
