All fascinating stuff, I am just to go on holiday and will ponder the sage advice give !
In terms of who did the mag, it was Vintage Magnetos in Bristol, the guy who rebuilds them is an ex Lucas employees, been using him for many many years and have never had a problem with any magneto he has done for me (he is super fast & very reasonable too !), the mags are always thoroughly tested on a ex Lucas test rig for an hour before he passes them on.