MC maris gone bad

Just chop the sluggy / holey bits out, the rest will be fine 

If it is slugs then next year try that nemaslug product as I found it worked well.
Slugs are a real pain. You should maybe lift them all and store the ones that are ok before the slugs munch more holes in them.

Good advice here -- get them out of the ground asap before the slugs eat any more.

Do remember that you can only store ones that are undamaged, even though damaged ones are fine to eat when 'trimmed up' 

Here in Glasgow, our potatoes this year are virtually slug free for the first year ever!!! We think that the hard Winter and dry first half year has killed off the keel slugs.

same here  although this year I grew varieties that seem to be more slug resistant than others (I'll NEVER grow King Edwards ever again!!!) but still, I've been so pleased with this year's harvests - downside, yields have been down due to the dry spell but at least I haven't had to toss any away due to slug damage.