We finally had this inspected by a specialist today (from Timberwise) - he observed that the subfloor is damp due to poor ventilation, but that hasn't yet developed into anything serious, so more airbricks is the likely way forward.
He also noted that our perception of damp in the actual room was due to excess condensation. There are a lot of cold/dead-air spots in the room where we have mould forming on wallpaper behind cabinets etc. These are typically north-facing external walls.
Companies like Nuaire were mentioned, so I've been doing a little research. PIV products like the Drimaster 2000 or 365 seem widely used, although opinion is mixed. I guess I'm looking for guidance on whether installing one of these is a worthwhile step. It's not a small house - 4 beds, 3 recept, 2 bath - and I'm slightly dubious that one of these venting into the landing could make a difference in the downstairs lounge. On the other hand, I'm not averse to the expense if it could be beneficial. Equally, are there alternative solutions that would be better?