Thank you maizie.
I stopped looking at this thread after I revealed the name of the fruit, but I'm touched by the concern of the grans who thought that maybe I'd had a little nibble of the fruit and suffered some dire consequence!
The tree is growing on some rough ground in what used to be an orchard, which is why I thought it's fruit might be edible.
Interestingly, when I cut it open to look at the seeds, the flesh oxidised almost immediately. Apparently it contains lots of tannin so probably tastes awful, but I didn't try, honest!.