[helijeli]

Actually that makes sense Dave. I've been reluctant to position the collective at 50% as you say, thinking that with whatever is wrong, the motor will spin up suddenly and definately strip gears. As you say, moving the stick gradually is probably what's causing unpredicable spoolups whereas I should use your method. I thought it strange that both ESCs were doing the same thing and I did check the ESC setting for Soft Start (I'll disable Governor mode as you've suggested).
Looking at the Heli Manual, I've worked out how to change the stripped gear and Northerner included a spare with my Heli. Just need to find/purchase the correct allen key for the gear shaft.