Paddles,
There is defenitly a Rocking Horse in that line up, I'm sure of that!
I think you are right. My bet is on the plum. Nectarines and peaches have normally elongated and more serrated leaves.
The leaves in your photo's are more rounded and have more rounded edges. Also the flowers are tiny. Again a plumb traite.
A lot of nectarines and peaches are grafted onto a stock called Myrobalan (Prunus cerasifera) which is a plum stock. So it could be that you have that.
Anyway, I'm sure you will be able to make something out of it
Hope this helps
Cheers
BoNZai