Potato dilemma

I only dug my Sarpo Mira back in January but had no problems when boiling them.

Is this purely not a case of over boiling?

No: The outside of the potato is breaking up whilst the inside is still not cooked. The potato ends up as sloppy mush.
But you do raise an interesting point. I always lift potatoes when the folliage starts to die back, to minimise slug damage. I don't know if letting the plant die back completely has anything to offer.