Thank you Grant and EJ. I looked up Thornapple and Datura on Google after reading your posts and am in no doubt that you have identified it correctly.
It would seem that all parts of this plant are highly poisonous so I will need to get rid of it before my young grandchildren visit in a couple of weeks. It is supposed to have a beautiful trumpet flower so I am hoping it will produce one shortly before I dig it out and dispose of it. :D I wonder how it got there, especially in with the potatoes? Maybe a migrating bird flew over and deposited the seed. :-\
Muddy boots, I have just seen your reply when I was posting this. Having looked up Grant and EJ's names for the plant, I am now certain it is Datura, commonly known as a thorn apple.
Thanks to you all.
Pauline