If you have left the potatoes in the ground, then I would lift them as it is the keeled slug that would be eating them.

Goodness your potatoes needed to be out of the ground a month or more go! Get them out!

I know that some gardening books recommend leaving some root crops in the ground, but I bet taters aren't one of them!

You can put the dug up potatoes back in the ground for storage, but that takes a bit of ground preparation to make a 'clamp' as it is called.

Parsnips and carrots you can leave in and take as you need, but not potatoes.