What you have on your spuds that you're calling grey mould is almost certainly potato blight. Now you have cheerfully demonstrated some lack of knowledge, so kindly don't tell me that I don't know as much as I think I do. I've been growing vegetables (and flowers) for almost 50 years and it so happens that I have a degree in botany (which included extensive work on plant pathology)...

Blight either comes in on the wind or has overwintered in your garden (or in next door's garden) on "volunteers" (potatoes that you missed last year). If you see those regrowing you should pull them up straight away. The minute spores can't overwinter as free spores in the soil. Washing seed potatoes before planting is probably useless. Seed that is carrying blight spores probably wouldn't have made it anyway, and, in this country at least (yours too as far as I can gather), seed potatoes are raised in northern areas free of disease and must be certified disease-free. Tomatoes can suffer from potato blight too, though the plants can often grow through it. Not so with potatoes. If you get blight early you'll lose most of your crop. If it strikes when your spuds already have good bulk, the best thing to do is to cut off all the tops (which you can compost: the spores won't survive) and leave the spuds in the ground for a couple of weeks. Most of the spores will wash harmlessly away down the sides of your earthings-up. There's no organic solution to an attack of blight. Get the spuds in early, grow first-earlies or second-earlies and keep your fingers crossed!

Blight on leaves looks like brown patches that spread very quickly over your crop. Each brown patch has a whitish fringe - that looks just like grey mould. :-)