I think that you can get mildew remover in the supermarket but realistically you will never remove all of it! If you are replacing them and they are in a damp area you should get PVC blinds in the future which can be cleaned very easily.
Thanks guys. When I moved into the house these blinds were there. They are a lovely quality so I decided to keep them but the mildew problem has got worse and to make it even worse again they're in a part of the house that doesn't get sunlight.