Been away for a while, but thought I'd post an update to thank everyone for their advice:
Things I have done:
- I bit the bullet and paid for someone to inject a chemical damp proofing course (having seen this, I actually think this is well within the scope of a competent DIYer. The chemicals are not expensive and more modern creams are injected into the mortar not the brick)
- I replastered the wall in a breathable plaster.
- I have not yet painted the wall as I am giving it as long as humanly possible to dry out. This means my home currently has a 'rustic charm'.
- I have checked under the floorboards for signs of leaky pipes. Nothing obviously wrong there.
- I have been around with a pointy stick to check the air bricks.
Other things that I will do in the future:
- eventually I will paint the replastered walls because my other half doesn't like living in what feels like a squat that we have to pay for.
- The house needs some ventilation generally. We noticed the plaster is taking forever to properly dry out. And a damp tea towel put on a drying rack is still a damp tea towel the following morning, so this house is screaming for some ventilation. When we next have some work done, I'll install some, which should hopefully help prevent future damp.
Thanks again everyone.