Ventelation is key - but if you've left a gap that's the first bit covered, If it's a cold corner, then it'll be where the condensation first appears. Either warm it, or dry the air.
As suggested, dehumidifier helps massively (a proper one, not the block of salt type!). I recommend ebac ones as I've run one in this damp seaside cottage for years just left to itselft on smart mode but they are noisy. Certainly not something to run overnight near a bedroom!
www.ebac.com/dehumidifiers/
Other things that have made a massive difference to this house is insulating wallpaper. Ignore all the rubbish about it making the room warmer etc (4mm of polystyrene is a tiny increase in insulation!).
We used www.erfurtmav.com/erfurt-mav-wallpaper-products/insulating-wall-coverings/insulating-lining-paper-graphite-plus in the worst area of the bathroom, and it's completely stopped the condensation by making the wall surface warmer. Needs overlining with lining paper (I'd recommend the fibreliner from the same company). Available from toolstation etc, so easy to get. You also need their special paste ideally. It's a massive faff, but makes a huge difference in real problem areas. even just a few strips on the wall behind a wardrobe can help.
Final thing that really helps is the correct paint. Wickes do a white antifungal that's ok, but the best stuff is the Zinsser perma-white.
Not cheap, but great stuff. Confusingly, although called perma-white it's tintable to most colours. I'd recommend
www.decoratingcentreonline.co.uk/paint/specialist-paint/damp-and-anti-mould-paint/zinsser-perma-whiter-interior-matt-colour-match.html
as a supplier - email them for advice, really helpful! Also decent prices for colour mixing it.
It's lovely paint to work with as well :-)
Both Ebac and Zinsser are UK companies as well I believe, so nice to support. Decorating centre is a smallish shop making a go of it online (and really smashing it IMO!).
good luck :)