G-BBYM.
I used to work on that aircraft in the 1980s at Glos Air Hurn. this A/C was sold to Bae along with G-GLOS for coms work. One based at Woodford and the other at Brough.
I don't think I've got any wiring diagrams, but if you are going to get the engines turning on the starter motor please observe the duty cycle or you will wreck them. They are very small and get hot quickly.
From memory it's 3 x 30 secs. on with 1 minute off and then 1 hour cooling.
Good luck.