Fleece is pretty useless against frost I find. When you earth up, make sure you draw the soil right over all the foliage - it looks as if you're killing the spuds, but you're not.
We're forecast -7 tonight, and have had snow since yesterday, so I'm pleased we're a bit behind with preparing the tattie patch.
Keep a record of your weather over several years, and eventually you'll arrive at an average last and first frosts date for your garden. Then plant your tubers about 2 weeks before that date.
They'll keep on growing in spite of the damage. You could always try digging up a couple to check what's going on below, to set your mind at rest