Can we have some photos Paul, I am thinking you have Lucas RITA electronic, They can be tricky !!.
I think they fitted them to Nortons ?.
Don't kick the engine over without the H.T. leads on, Or it will BLOW the magic Box.
I have that set up on my Twin, Mine works on 12v battery but uses 2 6v Coils !!.
I did break down once, Faulty coil, And all I had with me was a 12v coil, It got me home !!.
The distributor is an alloy casting with a polished alloy cover ?.
Have you got the old battery so that you can see if it's 12 or 6v ?.
Maybe take the head light bulb out and see if 12 or 6v ?.