Can anyone ID this fruit tree please?

[barefootgardens]

Wow! Wasnt expecting so many replies, thanks very much for your help!
Liking the idea of making wine from them

[cocobelle]

I thought under ripe greengage, but may well be mirabelle, either way if you have a tree of them you are in for a treat!

[Sinmara]

Agreed, looks like a Mirabelle - I love them :) Back in my childhood I practically lived on the Mirabelle tree on the neighbouring empty ground - I did all my homework there, reading books, etc *chuckle* And in summer/autumn I had the added advantage of having food right under my nose!