Green Willy..I don't understand something..Your appartment is 20-30 degrees? Aren't you freezing???LOL..I keep my temps set at 68F..If you're talking 20-30F it's a wonder you have any plants living..(S) there has to me a mistake.

Since there are holes in the leaves, have you checked for insects? Look for common bugs like spider mites, mealy, aphids and scale..Any sticky leaves?
Calatheas need a good amount of humidty. Humidifers works great in winter, and they're good for ppl, too. Daily mising of leaves helps bring up humidity. Water trays are another option..
All my Cals and relatives are about 5' from a west window, near north..I water only when soil is slightly dry. These plants do NOT need overfeeding..another reason foliage can brown.
In the meantime, check for pests..they cause all sorts of leaf damage..good luck, Toni

Thanx for the reply toni,
as i live in the uk i meant 20-30 centigrade so i'm sure its hot enough. I have quite afew plants and they seem ok, i'm looking for insects but cant find any,
another option could be the water in our area is hard .
anyway I will percivere and crack this nut
kind regards nick

Green, Silly question, but what's the conversion of 20-30 C to F? LOL..Sorry.
You don't want the room too hot..In fact, I keep my Cals in a cooler room in winter..Too hot and dry and plant declines. Toni