Wee problem.

[tiny soty]

My hamster has constantly got matted fur on his bottom and tail, it isn't wet tail as there is no diarrhoea and he is fine in himself. I think the problem is he is weeing in his house and on his wheel then sitting in it. I give him a good diet and a variety of treats, but he continues to wee and poo enough for 2 dwarfs!

[souffle]

Is it on his tummy at all? It could be from the scent gland and this can make them quite greasy and stain the fur.
If it is urine then it is not good for him to be wet as he could get urine burns on the skin.
I would stop using an enclosed house and just give him deep bedding and a huge heap of ripped tissue to nest under.
They tend to wee more if you keep cleaning them so sometimes it is best not to keep washing the wheel. I know it sounds horrid but once they have it nicely scented they tend not to do it so much.
He may however have some illness causing him to wee a lot. Kidney disease, urine infection and most commonly diabetes can cause excess weeing and drinking so if you suspect this you should see a vet for urine testing for sugar.
All our dwarfs have wee'd on the wheel. It is very common!