Musty smell - damp chimney?

Southdunedain

Hi, 
We recently moved into our first house - a '50s semi. Since we moved in, there's been a musty and unpleasant (kind of sweet) smell in a first floor bedroom, which we believe to be damp. We thought it was a piece of furniture we'd brought with us, but having moved that out the smell remains, albeit frustratingly undefinable/untraceable. 
Our attention has turned - basically by virtue of there being no other obvious candidates - to the chimney breast, which carries an active flue from a wood burning stove downstairs. There's a vent in this chimney breast which smells/looks fine. The survey noted that the chimney needs capping, so I'm aware this is a damp risk. However there are no visible signs of damp anywhere on the chimney breast - including downstairs or in the loft, where the brickwork is obviously exposed - and it doesn't feel damp as far as I can tell. The stove downstairs draws well and the flue looks reasonably clean. 
So, I'm stumped. Otherwise the room has clear walls and a wood floor, and nothing shows any signs of damp or disrepair. All the remaining furniture is wooden and came with us. Nothing relevant was flagged on the survey apart from the chimney cap; the surveyor thought the property throughout in an exceptionally good state of repair. The smell isn't in any other room. 
So, a) am I missing something obvious? And b) if it is the chimney breast, do you have any suggestions who would inspect/diagnose/remedy potential issues of this kind? 
Many thanks! 

Grenage

I assume you have ruled out the floor and the void below?

FreeBear

If you are regularly using the wood stove, it will be heating up the chimney breast up stairs quite nicely - If you can pick up an infrared thermometer somewhere cheap (around £10 for a new one from Lidl), you can check to see how warm the wall is getting.

Mine is often around 25°C which is more than enough to keep any damp at bay - I do have the unused pots capped which stops rain from getting in.

Chickereeeee

Grenage said:

I assume you have ruled out the floor and the void below?

This is often the culprit on a ground floor: less usual for first floor. It could be the carpet (if you have one) has an 'historic' smell from something that has now been removed, or it could be something under the floorboards is not right. Lift a floorboard and have a sniff: you might be able to get an idea from sniffing around the skirting board first.