Stored Potatoes problem - are they frost bitten?

Hi

I had a large Potato (maincrop) harvest.  These were stored in hessian sacks in the shed.  We are now having a problem of grey and back spots on over half of them.  Has anyone got any advice or know of the reason?   The shed will almost certainly have got frozen, hence thinking this is the problem.  We had no problems before the cold weather before Christmas began.  They are a mix of Maris Piper and Desiree if that helps.

This has happened to me, the Clara that was stored in hesian have been frosted, only the outer ones, the ones in paper sacks were ok. I chuck mine in the wheelbarrow to sort and re-store and compost the soft ones. If we get another cold next year then I will at insulating the shed.