Thanks for the info. The foliage from the potato plants has all but died away, so I've removed what's left.
Here's an image of a mildly effected potato with raised blacky/brown 'warts' on the outside of the skin. The potato is still firm, and once peeled can be eaten.
The other batches are a mixed bag, some fine others with warts, and some a bit squishy!
I would t say under watering has been an issue, the climate here, damp, heavy rain, humid has been far from ideal.
Oh well! I think I'll go for potatoes in pots next year with fresh new soil to be on the safe side.