Seed potatoes!? Are these worth keeping!

I wouldn't bother with them, they could be any variety, new, second earlies or maincrop. The other thing is that they could be carrying pests and diseases. I'd recommend you buy proper seed potatoes, as then you know for sure what you are growing, they are also certified free from pests and diseases so you'll stand a much better chance of getting a cleaner crop and bigger yield. If you go down the supermarket route you could waste a lot of time growing a crop that succumbs to blight (for example), basically it's a gamble - why take the chance when seed potatoes are relatively cheap?