I know it's a common fault with NC29s.

What is the best fix for this - when I say "best" I mean "Most Cost Effective" :D

May I add I have the technical knowledge of an Amoeba.... :)

There is 2 ways...

Buy a new engine with a good gearbox.

Or engine out split the casings and fix the worn dogs causing the problem.

Personally I would go for the first option, either way you are taking the engine out.