Ok, a couple more test flights done.
Each stall was produced as a snap roll induced at 200 ias, by about a 20 deg nose up, 30 deg bank, then sharp back stick and full rudder together.
Alt was 12 - 14k, fuel 50%.
First 3 times I could recover without difficulty. I tried with combat trim on and off. Recovery slightly easier with it off.
It was stable enough that I was able to complete the P47 "flip around" manouvre under full control. (Might be worth working on for the fans).
I was about to concede I had it all wrong when a flash of thought. I fired off the ammo until about 100 rounds of cannon remained and tried again.
Persistent unrecoverable spins. Each time I tried I got the same result. It might be that reducing the ammo load brings cg aft enough to make it unstable? Is this sort of accuracy modelled? Is this something that needs a tweak or is it a feature? I would be pleased to hear if others get the same result or if it's something I've got screwed in my setup.