My thanks to all the useful advice so far - it seems a lot of people out there have had similar problems in the past. For those of you who are interested in this saga, I have copied part of an e-mail below, which I received from Ken Targett at Brightspark Magnetos, ( ' KenF ' of this goodly Forum ), who gave my mag a good going over. Those of you of a nervous disposition should look away now......... !!!
" Hello Norman,
Your magneto parts arrived safely, and I have now checked them out. The points of note are:
• The magneto body was fairly well magnetised. I have managed to increase the magnet strength by 11%.
• The pick-up brushes are indeed too soft for a twin-cylinder magneto. We can supply a pair of the appropriate grade if
you wish.
• The armature appears to have its original windings and condenser.
• The condenser should ideally have a leakage resistance in excess of 2000 megohms when tested at 500 V. Your condenser's leakage resistance registered 0.1 megohms when cold, and 0.0 megohms at 50 deg. C. Because it is so leaky, it failed to give a sensible reading of its capacitance on our capacitance meter.
• There is a lack of continuity through the high-tension winding, i.e. there is a break in the very fine wire of the winding. Despite that, a broken winding can still work, with a spark jumping the break in the winding as well as at the sparking plug. Not at all satisfactory, because the extra spark burns away the insulation, and also erodes the copper wire and makes the break bigger. I have tested the windings on our coil tester (click here for details) using a 5.5 mm 3-point standard test gap. The low-tension break current required for consistent sparks was 1.7 amps when cold, and 1.9 amps when heated to 50 deg. C. Those figures aren't terrible. So, if you only need this magneto for very occasional use and it doesn't matter that its performance will fall off over time, then you could use the original windings. I can't give any guarantees at all about how long the thing would last. It will be far preferable, of course, to have armature bobbin rewound. We don't do rewinds ourselves, but send them out to a time-served rewinder who guarantees his work for one year. The turn round time is about three weeks.
• I have also checked the slip ring and pick-up mouldings on our tester for that purpose (click here for details). The slip ring, which looks like the original, tested good, cold and hot.
• The pick-up mouldings are of Taiwanese manufacture. Both tested good when cold. However, after heating to 50 deg. C, connecting one of them to the test circuit required 35% extra LT current in order to maintain the circuit's spark, and the other pick-up required 40% extra current. They are therefore very leaky when hot and need replacing. "
The slip ring referred to is not the cracked one I found originally, but a NOS replacement I found hidden away in the workshop.
The main problem, of course, appears to be the condenser, as suggested by several of you at the start of this thread, but I would never have been able to check the plug caps, and they obviously are not helping the problem. My fault for buying cheap imported rubbish, I suppose. Anyway, good to read such an in-depth analysis of the faulty mag, something I would not have been able to accomplish in my limited workshop at home.
So, good news then. All I have to do now is wait for the bill to hit the doormat.......... Oh well, beans on toast, and bread and jam for the next month, I 'spose !!!
Still haven't plucked up the courage to remove the A10 mag yet, John, as a group of us are still riding out regularly before the summer disappears, but thanks anyway. My thanks to everybody who replied to this post - nearly everything you all suggested seems to have been covered in Ken's e-mail. It pays to talk to the experts. 
( Should have done that in the first place - Doh! )
More to follow, probably
Norman
while you are waiting try the other bikes mag my then get a good nights sleep in the mean time john
