shellingpeas is right, they are the potato fruit and are REALLY poisonous but the tubers will taste lovely. and yes potatoes are related to tomatoes. So nothing new to add to SP's post - sorry! but just wanted to back them up! Enjoy your tatoes, we've just dug ours up and they are gert lush
I grew up in New Zealand and we used to get potato fruits on our plants frequently, which was how I knew about them - must be because it was warmer maybe? I never knew they were a rarity in the UK, I have learned something new today!
interesting that they are common in nz....it wasn't even mentionned in any of our growing veg books....glad you told me they were poisonous, i was going to see if they ripened on the windowsill! mind you atm they are very unappetising looking