Thanks so much for all the replies! I don't think it's slugs, it doesn't look like it. And I've never been able to find any all the way up the plastic barrel. But at one point I had a big compost bin (empty, just delivered) next to them for a few days and when I moved it, a gazillion little white flies were disturbed and that's when all the little holes had appeared as well, and now there's always a few around there. -Flies I mean.
I don't have a hose and if they don't destroy the actual tubers, I think I'll just leave it. I'll have a look at the cabbage white fly posts.
Thanks again everyone who replied!!