It's been said before, but a good way to hold the armature still Slymo is to remove the earth brush from its holder, insert a hard wooden or soft metallic bung, even a too-long carbon brush (snug, so it won't drop into the mag, but not so snug you can't get it out again) and do the thing up till it nips the armature. Works a treat, for me anyway, when dealing with one of those 'won't sit still' ones.
However, that's easy compared to BillyB's possible ailment . . . they do break sometimes sadly as Orabanda said. At least you've put yourself in the right hands!
I have a theory that the vulnerable ones are the early 15mm drive end diameter ones, when carrying an ATD. Only observational, but the broken ones I've seen - there have been a few - have all been the small shaft diameter ones, with fixed camring and ATD. I consequently am reluctant to use an early one with an ATD myself.
The trouble first arose, or so legend has it, at the TT, on a Porcupine, which had shaft failure when in a leading or podium position . . . and Lucas reacted by upping the diameter at the drive end on all versions thereafter. That might be apocryphal, and there are no doubt other legends, because they already had Racing mags with one piece stainless steel drive ends in the K series, also in NR1s I think . . . but the point remains . . . early ones are more fragile.
Some repairers will machine out the earlier bare housings to take the later armature, bigger bearing & oil seal etc - but it's a bit of a labour of love (DAMHIK) unless you set yerself up to do it on a regular basis.
