@WombatChocolate
How often do you have the heating on and windows open through the flat, especially in winter? How many hours per day? Do you dry washing on radiators or in flat?
Very often these are causes of damp and tenants or owners have caused it themselves rather than there being a structural issue. people don't have their heating on enough (to save money) and don't like windows open as its draughty, but the reality is, those need to happen and whatever curtains you have, the issue will return withour good regular ventilation.
That takes me back to my discussions with the local council when I lived in one of their flats.
I sat in my bed one late December night with my breath sending out huge clouds and I measured the room temperature 3 foot from the too hot to touch radiator. Minus 4 Celsius. Water pissing down the outside walls.
I was repeatedly told that it was because I didn't have any windows open, because I was cooking too much (two cooked meals a week as we ate at work/childminders), because 'teenagers are disgusting sweaty creatures' (had a nine year old and a three year old), because I was at home all day breathing (at work fulltime), I hadn't put the heating on enough, (try installing more than a single gas fire in a three room property then), etc, etc and I should make use of either the secure drying area outside or, if it was being used by the other residents, go the launderette round the corner to dry clothes, as I should remember that it was a breach of my tenancy to dry clothes on the balcony.
They'd turned the secure drying area into a bin shed approximately seven years before I'd moved in and the laundrette round the corner had been levelled, along with the rest of the row for them to put up a block of flats in the last five. They should have really known this, as they had tried to evict but had been ordered to rehouse all the tenants of the flats above the shops in order to start demolition.
I finally found somewhere else to live and gave notice. They told me that I would have to pay to dryline the inside because I'd caused a structural issue through letting it get so damp inside. I requested this in writing, together with an estimate/bill so I could seek legal advice and confirmation of the reasons why they had been taken to court by five other tenants in the block immediately prior to drylining their flats, replacing their furniture and making compensation payments to each - at which point, they forgot all about charging me.
After I moved out, they apparently failed to let it to anybody for a year because the Fit Homes Standard applied to new tenancies, then, rather than lining the external walls, they glossed them and removed the opening windows to replace them with non opening ones that had passive vents in them. so they reduced the ventilation and increased the likelihood of condensation.
Anyhow, an electric humidifier will be useful. Net curtains and IKEA curtains will be better visually and because you can clean them and they'll take out draughts (absorb lots of water too, but we won't mention that). And a polite audit trail of you requesting repairs because the LL's inaction is resulting in the damage will be useful for making sure he doesn't accidentally on purpose try and charge you for damage.