Same as your skimmia - too much water. I also see mildew on the wooden planks - must be rainy there.

Powdery mildew on Geranium is very much to do with airflow round the plants and damp weather, or conversely, being too dry at the root. Try the milk treatment (2 parts cows milk to 8 parts water) or a fungicidal spray, but to be honest, I'm not sure there's not something viral going on as well, with that yellow banding/streaking. If the healthy leaf on the right is a more recent one, then there's hope for it, and I'd be inclined to remove the yellowed ones if it is. I wouldn't put it in the cold frame, that'll reduce air flow even more.

Ok thanks Bamboo. We can only try.

Bamboo - just wondering do I apply the milk treatment just the once or everyday for a few days?

As often as you like, but I'd try it once initially.