What is this and who can fix it?

[peachsnapz]

Please can anyone help with what is causing this and what I need to do to fix it?
The room is an upstairs bedroom, gutters have been cleared and are apparently not leaking. Roof was re felted(?) a few years ago so shouldn't be leaking.
It is slowly getting worse.
Thanks in advance.
😊

Its only ventilated by us opening the window which in the winter we obviously don't do enough as get terrible black mould. Ok at the moment though as window open all the time. I will definitely get someone up in the loft to look.

It's clearly a ghost

Definitely look into getting a decent extractor fan. Will rid you of your mould issues even if it doesn't help with the wall!

Bathrooms need to be vented all year round. I think we've hit the nail on the head as to why you're having problems in the bathroom and adjoining room. You need someone to come to fit an extractor fan wired into to come on when the bathroom light switch is turned on, and with a built in timer so that it stays on for a while afterwards. They're not just to get rid of "smells", they're to get rid of the humid damp air after baths and showers.

In addition to an extractor fan then try and heat the bathroom a while after a bath/shower to help dry the room. It seems crazy to warm the air and then use an extractor fan to vent it outside, but it will dry the room out far more effectively than if the room is cold. A dehumidifier on the landing or in that bedroom is also worth considering.

Thanks. Might be a stupid question but who would do that? A handyman, electrician...?

With it being bathroom electrics, an electrician will almost certainly be the best person for the job. They're accustomed to fitting this kind of small appliance all the time. Not sure a handyman or plumber would be allowed to do the electrical side of things with it being in a bathroom due to the risk of water causing electric shocks etc if not wired in correctly to standards.

We've had things like electric showers, fan heaters, standalone water heaters, etc., fitted to kitchens and bathrooms over the years (different properties), and the plumbers have never done the electrics themselves - always called in an electrician mate to do the wiring.

Electrician. They should vent the fan to an external wall. You might need a builder to make good and you can have the fan in the ceiling or the wall but you must vent to the outside.

The wall looks like damp in a narrow location so a pipe dripping? Definitely get in the lift and look at anything above that wall.