I am sorry for the delay, been away on holiday. Its raining today so will take a picture of the wall from outside tomorrow and post it for the follow-up. Thank you.
There is combi boiler in a cupboard next to it but the water pipes are going up and outside the wall (to the best of my knowledge). Not sure about the gas pipes.
I don't have cat.
I removed bookshelf to give it a good clean and reorganise the room a bit and noticed yellow stain on the wall. I am not sure what has caused this or how old it is as the shelf has been there for over five years. How can I go about investigating it? Thanks