Water coming down chimney? Selling house!

[turquoise50]

I live in a Victorian end-terrace. The chimney breast in my lounge and the wall to one side of it have become very damp, especially (but not only) when it rains. The wallpaper is peeling off up to about 1m high and the plaster looks to have gone friable in one area at least.

My neighbour is a builder and so far he's done the thing where they drill holes in the brick and inject a chemical - it was already there but he's extended it one row higher - and he's also re-pointed a large area where there was previously a shed attached to the house. He's only done it up to about head height because it looks ok above that.

He did it at the weekend but then yesterday it rained torrentially and last night the wetness seemed worse than ever. Honestly sitting in my lounge felt like a sauna, there was so much humidity in the air. So I’m now thinking the water is coming in from somewhere else and what he's basically done is block the partial escape route which it had before through the gaps in the brickwork.

The obvious candidate for point of entry is the chimney, especially as I’m sure I heard something fall down it a couple of months back, although the problem pre-dates this. There are also some much much smaller damp patches on the chimney breast in the bedroom above the lounge. I’m assuming that whatever kind of cover the chimneys had on them (there are four, I think) has come off or partially off.

My question is: has anyone any experience of getting something like this fixed and could give me even a vague idea of how much it would cost please? Covid has sadly destroyed my livelihood, and the small amount of money I have left is earmarked for a course which I badly need to retrain for a career change. I've already paid the neighbour several hundred pounds for the job he did, which I don't begrudge because it really did need re-pointing, but I can't afford to spend any more unless it's guaranteed to fix the problem.

To make matters worse, I need to sell the house next year! Most houses round here sell to BTL landlords who tend to gut them, so properties in move-in condition tend to take longer to sell than ones in need of refurbishment. Which makes me wonder if it's even worth fixing or whether I should just take a price hit instead. Can I even do that? What would be the relative costs?

[Mintjulia]

I had this with my last house. I had the flashing replaced around the chimney, the chimney repointed because it was very weak and the chimney -which was not in use, capped with a side vent.

I also had a vent put in the chimney breast on both ground and first floors. Outside of London, it cost about £1500.
It took 6 months to dry out properly so I could redecorate.

[turquoise50]

@Mintjulia Thanks, that's helpful. The bedroom above already has a brick-sized vent in the breast which is possibly why there are only very small hints of damp there. Sounds like my next move is definitely to unboard the fireplace and see what's behind there.