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

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!

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!