Ian, that is the worst case of white spot (ich) I have ever seen! The important thing here is to treat the whole tank as soon as possible. Each of those spots will release thousands more parasites into the water. ONLY when they are in the water can they be treated. Get hold of a reliable treatment and follow the instructions carefully. As Brad M says, raise the temp steadily to around 80F - this speeds up the development of the white cysts so they burst sooner and release their nasties into your treated water, where they will die. It is important you complete the treatment to prevent re infestation, but remember, the fish may still have parasites in them which will hatch later, so a second treatment may be necessary.