Any ideas what this is?

Looks like scab ( Streptomyces scabies . )

Have you tried cutting open that potato and look for discoloration or other symptoms?

No I didn't cut it open and don't have it now. It is from a community project. I will cut open another one if any more come up like that. There are quite a few in one spot of the garden that were like that or with the knobbly lumps even worse. I didn't think it was scab because it is so 'lumpy' rather than 'scabby'. It doesn't really look like scab as I know it. I wondered about wart disease?
Happy to be persuaded its scab though ......... that is a lot less serious.

https://spudsmart.com/potato-w...

Potato wart is caused by the soil-borne fungus Synchytrium endobioticum. ... endobioticum pathotypes evolving and the increased risk of dissemination by the regional and cross-country trade of infected tubers. Although potato wart poses no risk to humans or food safety, it causes tubers to become disfigured.

The reason I asked you if you cut it open is it can provide very useful insight about the disease. I suspect this potato was grown in a soil contaminated by Root knot nematodes (Meloidogyne sp.) but you can confirm that only by slicing the potato open.

The most common Potato scab form in Europe by far is Netted scab, which looks radically different.
Powdery scab in Europe is relatively rare but unfortunately very often associated with Potato mop top virus (PTMV), which is easily spotted on foliage.
The Wart disease manifests itself in different form: by the time potatoes are harvested the warts have already taken the diagnostic "cauliflower" shape and blacken very quickly when exposed to air. This doesn't appear to be the case.

Ok Thank you. It seems like the root knot nematodes are a likely cause. I will look into that further. Many many thanks