[Ivah]

Looks like a Bullace (wild plum) to me, fruit green/yellow rather like Greengage, makes a fantastic tangy jam.

[joyfull]

I would say wild plum - or a plum tree that has reverted to being wild (grown from the original base that your tree was grafted onto).

[8doubles]

Looks like a Bullace (wild plum) to me, fruit green/yellow rather like Greengage, makes a fantastic tangy jam.

Wild bullace are normally purple in colour like a large sloe or a small damson.

If they go a bright yellow they could be mirabelles.

[PennyS]

Looks quite like my greengages?
Anyway, I'm sure whatever it is it'll be lovely to eat 

[Spana]

We have a tree in a hedgerow with just the same fruits.  They are like tiny golden plums.  When they start to fall  badgers come and eat them and leave us piles of poo full of the stones .   

[Paul Plots]

Still looks like our nectarine fruits

Nectarine in bloom:

[Ivah]

8doubles - I beg to differ

http://www.brickfieldspark.org/data/bullace.htm