If you need a dehumidifier full time then that suggests a more serious
problem.

owt much we can do about it at the moment sadly. we only need it running 1-2 hrs in the mornings though, and the humidity has massively improved

What sort of heating do you have ?.
Do you use showers or take baths ?.
Do you do a lot of cooking ?
Do you use a tumble drier indoors without a vent kit ?.
Does the property have cavity walls that are uninsulated ?.
How much insulation is in the loft ?
Are you in an exposed area as defined by building regs ? (i.e.
somewhere on the west and south coasts with more driving rain).
Does the property have cavity walls or solid walls ?.

-we have a combi boiler system heating the rads
-mainly showers, sometimes baths. Shower is in downstairs extension bath upstairs on opposite side of house to this bedroom. Shower room has its own fan vent and we keep door closed. I think the bigger issue with condensation is inside our old secondary glazing in 3 of our windows downstairs (the rest are double glazed). But again not much we can do about it for the the foreseeable future besides opening windows, wiping down and running the dehumidifier. Kind of running on a shoestring out of necessity atm.
-we cook about as much as a normal household would? :P and we have a cooker hood fan that is always turned on when cooking.
-don't have a drier. But we do dry our clothes indoors most of the time.
-no idea about the cavity walls tbh. As far as I can tell we have decent insulation, house traps warmth pretty well.
-We're in Cheshire, there's 90-95% humidity in our area pretty much the whole year. It's rain central here.

Never been in the loft myself, no idea what's up there. Don't even have a ladder for it. A little scared of what I'll find lol! But given several people have said to check the roof & loft for leaks, I'll have a look up there as soon as I'm able to. As you guys have said there doesn't appear to be moisture inside the plaster itself... so unlikely the vent is the issue even though it's rusted through. For now I think I will just reinsulate the gap and reseal with plaster, and will look into the roof again. Would painting over the rust be a bad idea? (sorry if it's a dumb question lol)

Not able to afford any major refittings at the moment so going to see if I can get spacers or something to allow airflow between the wardrobe and the room whilst still keeping it (mostly) closed so my cats don't terrorise my storage haha.

Again thanks for this guys, really really appreciate the help. You've been brilliant.