"Helen Deborah Vecht" wrote
There are some 'accidental' fruits in my garden, which I'd like to
identify.

I have eaten (and stewed) quite a few without ill-effect and they are
ripe now. They are yellow, about 1.8 cm in diameter and have a central
stone, measuring around 1cm.

The flesh is sweet and tastes just like a plum, the skin is rather
sour.

The branches of the tree which bears them have long (2-3cm) sharp
spikes/thorns.

They're obviously a prunus, I think. Are they mirabelles? Googling so
far has not shown me the rest of the plant. The spikes are vicious...

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

--
Sue