Despite describing myself as a newcomer, I have previously had a allotment, about five years ago. It wasn't successful because I wasn't able to put in the time.
I did plant some potatoes, maincrop I think. I have to admid it took me some time to harvest them and when I did they were full of black holes. For future reference, so I don't make the mistake again, where did I go wrong?
Various possibilities, the most obvious being worms or slugs, probably slugs. As you said, you didnt put in much time, so attention could have been a problem, also the condition of the allotment at planting time could also have been a factor.
I would suggest you start afresh, and good luck :)
It sounds as if you have a slug problem. The holes are made by young slugs. There are several answers. Grow a slug resistant varietie such as "Kestrel". The longer the spuds are in the ground the more chance the slugs will invade the tubers. So grow a varietie that is in the ground for the shortest possible time. These are growing times.
"rocket" first early check after 8 weeks onward
other first earlies ready 10 weeks onward
second earlies ready 13 weeks onward
early maincrop ready 15 weeks onward
late main crop ready 20 weeks onward.
another answer is to wait untill the spuds need earthing up for the first time then water the potato growing area with nematodes, these are minute one mm long "worms" that eat slugs.
Quite often, it also wireworms that make the initial small holes, then the slugs get into them and chomp away! Since wireworms are a problem on uncultivated land, maybe your spuds will get better as time goes on.