Ours used to look like someone had got a knitting-needle onto them, about 4.5mm holes, sometimes with the worm in but usually absent. Timing helped, ie they usually got into the early ones and left the maincrop ones alone more.I don't know! Dad said it might be he used to grow quite a bit of a garden so would know more than me about it but I'd already thrown the potatoes before he came to see.
The tops just died almost overnight so I dug some up and they had these holes in
Will try mustard next year