Ok how hard is it to repair? What is it that needs repairing? How much is it gonna cost?
Cheers in advance you clever people.
Chris. :)

hey up, your not alone sixth gear on my bike keeps jumping out of gear. i just bought a low mileage gearbox of ebay along with a new haynes manual. but this is my second time trying this and it was quite tricky the first time. but it was easier than i thought it would be.
or buy the bits and take it to a garage, don,t know how much it will cost? took me a good few hours/days but i,m not a mechanic.
it,s probably either the gear dogs or the slots that the gear dogs mesh into. apparently this is the most common fault with the 400 nc29 gearbox through iffy gear changes.
if your good with your hands, give it a go but remember you,ll need gaskets and other stuff if you try it.
sorry that this was no help what so ever, but hey

I've got a good gear box if you need one :)

or the gear selector fork.... they bend easy.

hello criss give me a call on 07766525812 and ill give you some advise