How worried would you be about this?

Look up leaks on Google.
Leaks onto walls and ceilings can happen due to blocked gutters. Leaves etc bunging them all up. Moss. Or damaged tiles here and there.
You do need to find the source of the problem to get it sorted.
Try your local Council website for a Trusted Trader and look for a handyman to do an initial check for you. He or she will not be free but a first port of call.

A common thing around chimneys is the lead flashing needing replacing. So one of my leaks was that my chimney needed a bit of lead flashing replaced and a bit of repainting.
Once fixed you can get some zinzer and put a coat on to stop the water mark coming through and rhen paint it as normal.

I had multiple roof leaks in my old house and all it was, was a single cracked tile and a bit of missing pointing. Dont panic. Get someone to have a look.
Can you see anytging obvious from the road, i could see one of my tiles was cracked when I looked.

OP a builder won't charge to go up and have a look, that is what they have to do in order to find out the problem and tell you how much it will be to fix it. They may take a photo on their phone to show you the problem.

Can you get into the loft yourself and have a good look around in that area. You might find that by now there's a load of soggy insulation which should be removed or the ceiling will take much longer to dry out.

It doesn't look too bad at the moment, not at the significant damage level at all and once the leak's fixed you should be able to just clean the mould and repaint. Use stain stopper paint to cover the water mark once you've treated the mould. Come winter and more rain, it will get worse so worth doing now whether your moving or not, eventually you will need to replace the plasterboard ceiling if it stays wet.

We recently had a lot of work done on our house and several persistent leaks were identified and sorted out. None of them were very significant, a missing tile her and there, some flashing that needed replacing.

I would ask for recommendation of a good local builder and get him round to look. He will need to get into the loft to look, and he may bring a ladder to go onto the roof.

Get gutters cleaned. Check for overhanging trees . Get flashing checked on any exterior. If it’s rain related it’s probably a leaky roof. Remember water runs to the lowest point so it might not be above your damp corner. Number 1 is your gutters though because water pooling over a weak spot will exacerbate it. Good luck, hope it’s not too expensive!

The gutters should be immediately below the edge of the roof so they catch the runoff from the roof, how do you mean they are lower? Or is that a room in the roof so there is the tiled roof above, which would presumably mean no access from below and it must be a roof issue.

I had this and it was because my gutters were blocked and backing up when it rained, forcing the water under the edge of the roof. It was a simple fix and once the ceiling had dried out properly it was repainted and no one would ever know. I would get a roofer out to have a look but it could be quite simple

It could be a gutter or a few tiles. Can you see the gutter from the ground and see if it overflows or leaks when it rains? Or if any of the tiles look wonky or missing? I assume that is an outside corner not an internal wall corner. The worst thing is if the timbers in that corner of the roof start to rot so I would get someone to get up there and have a look if you can't yourself. You will have to do something about it to sell the house.

Not necessarily. It could be raising damp from blocked gutter. When did you last have yours cleaned?

When I moved into my home the bedroom ceilings had been damaged by rain water. The water had been getting in due to damage to the flashing round the chimney. The previous owner had had the problem sorted - i saw the tradesman invoices and receipts.

I can't get up my loft either OP and i worry greatly about the roof, even though I had the roof replaced when I moved in. I send my son up there every few months when he visits, generally after we have had rain, just to check on things.

I agree with pp its a problem that you need to get sorted.

Is this a top floor room (below the loft space) if so I would assume the roof or guttering otherwise. that doesn't necessarily mean the whole roof needs replacing but it could be anything from blocked guttering to a slipped tile to anything upwards in severity.

I appreciate you may not have the funds for repair necessarily but I'd have thought the sensible thing to do would be to get a roofer/builder over to diagnose and quote. It's not like that will cost you anything and at least you will then know what the issue is. If you are able to get the work done then obtain several quotes.

It might just be a tile or two and not cost too much. As pps have said, any local group you’re in ask for a recommendation of a roofer. It will save money in the long run to get it dealt with sooner.

It shouldnt be too expensive.

See here for guide on price

https://www.checkatrade.com/blog/cost-guides/roof-tile-replacement-cost/

It could be the gutter but the way the stain looks it is more likely the roof. We had issues with both at a previous house, but both were easily and cheaply fixed. The gutter was blocked and not properly aligned, it was half a day's work to fix it. And same with the roof, it just needed a few tiles replacing.

We have always found our best builders/roofers etc by word of mouth from neighbours or friends so ask around for a recommendation.

For the stains, we treated it with stain stop and repainted.

You'll need to get a reputable roofer in for a quote OP. It's probably a loose tile or something not too horrific. After that it might need re-plastering and obviously redecorating. You won't be able to sell it like that.

Do you have any photos from the outside?

It's probably pretty minor but the water leak causing it needs to be sorted. After that, lick of paint and it will be fine.

Op this could be a very easily fixable leak, get a gutter/roofer guy round to have a look it could cost buttons to fix as opposed to thousands if
you let that rot all winter. At least find out what it is, don’t bury your head in the sand and don’t mess with water

Have you had your gutters cleaned recently? It could be something as simple as that. Also the gutter people can have a look and tell you if they see anything suspicious.