We have had a dehumidifier for two years, since we moved into a bungalow previously occupied by a man who never, ever opened a window. He said he kept the windows closed in winter to keep the heat in, and in summer to keep the flies out!
He wouldn't hang washing outside either, everything was dried on a rack in the smallest bedroom.
When we moved an old wardrobe from that room, we found black mould growing all over the walls and floor behind and under it, and the wallpaper had peeled off the wall and was crumpled up behind the wardrobe.
Mr J ordered our dehumidifier immediately, collected it the next day, and it's been worth every penny!
We've had every room re-plastered, and as every ceiling was covered in heavily patterned artex, all of these were done too.
I can absolutely recommend our dehumidifier. The model has changed slightly since we bought ours, but the new version is...
Blyss WDH-316DB 16Ltr Dehumidifier. We bought ours from Screwfix as the price was good and next day collection was very convenient. Screwfix Catalogue Number 368GY.
B&Q also sell Blyss brand products.
We're 'fresh air fiends' and have windows open all day, and open on night latches every night, but even so, some rooms get a tad damp, especially when washing has been dried indoors, even when on a heated airer (Lakeland Ltd). A couple of hours with the dehumidifier on in the evenings and the rooms are all at 30-35%, which is perfect for winter months. Summer should be 35-45%.
Please, please don't waste any money on the little plastic boxes which contain absorbent pellets and cost anywhere between £2 and £10 each, they will not help in any way at all!