I made crab apple and medlar jelly a while back. Daughter # 1 said it was the best thing she'd ever tasted
Although I don't know as much about this fruit as Zigs does, it looks like this ... and is a very slow growing tree. It belongs to the rose family and seems to look something like a rose hip and a bit like a little apple. I'm told they have a flavour that requires a bit of getting used to. maybe they taste better than they look.
Tetters is right, not a popular tree on account of the fruit being rock hard until the frost gets them. You can eat them after that as they go slightly rotten (called "bletted") and soften. Rats love them, left a bag of mixed medlars and apples outside, they had all the medlars and left the apples They taste and look like brown custard and are not usually sold in Tescos
We have a fruit similar--persimmons. They need a frost to be edible, and make a nice jam or jelly. We had them when we lived up north, but not here in Texas. I can't honestly say I miss persimmons!