Your post helps a lot thanks

As for British Leyland electronics

I see where you are going with that lol.
You probably already understand exactly what I meant,, but just to clarify...
With all 3 trigger switch wires disconnected and exposed, find the 2 that have no voltage between them. The other wire then should be the power in to the switch and it should show voltage when measured between itself and either of the other 2 wires,, all with gun switched on.
I'd have the gun ready to shoot, cocked, etc, since more then likely could be sensors to prevent firing with a empty mag/not enough tank pressure/safety on, not cocked, etc, or something like that, preventing the solenoid from firing till it thinks all is ready with the gun... Maybe not, maybe it has no sensors to prevent firing and will fire no matter what other gun parameters are..? But never know what precautions they engineered into the circuit.
Then try jumping the power wire to an individual switch wire 1 at a time, then remove and jump to the other wire and you should be able to see what wire triggers the rifle, and if it triggers upon connection or disconnection.... Anyhow, thats what I'd do if I had no wiring diagram..
Do they publish a diagram of the circuit, or is it top secret and only Christopher Steele knows..?
jmo