Help! Discovered that the waterproof cover over our lovely new rattan set isn't actually waterproof and half of the cushions are covered in horribly mouldy type stuff. I've washed them normally, washed them with some random stain treatment I found in the cupboard, now I've just soaked them in water and a bit of bleach for half and hour. Nothing is happening, any ideas please? They are made of polyester.