I have only ever seen a few tubers like this. I have no explanation for this condition but have managed to find an American site that deals with this. After reading the information, I am sure that like me, you will be none the wiser. https://cropwatch.unl.edu/potato/cracking. Good luck!

Thanks, Jimmy, I saw something similar to this suggesting it could be down to irregular watering. I am wondering if the use of manure to close to the planting period may have something to do with it. I added some bags of manure bought at the garden centre about a month or so before planting the tubers.

The west of Scotland is not known to be too dry and we cannot say this has been the case this season either. However, the temperatures have been poor for most of the early summer and many vegs have done badly.

Re-hydration does that. The potato's skin sets and then it try to grow again. Happens with apples too.

So, does this mean it is fine to use the potatoes evn although they have split? I am thinking that instead of discarding them, I could at least make potato scones from them.

There is nothing wrong with them. Perfectly usable.

Thanks, Owdboggy.