Hopefully i can help..
Has this been a sudden failure - was there any obvious problems before hand,noises etc before you lost 6th..
nothing obvious just suddenly dropped at 85-90 on A road then lost acceleration in 5th & 4th seems to have no other faults now except no 6th gear
also an oil warning flashed when i started the engine prior to breakdown but went out after 30 sec but is not on now
Hi.there..thanks for getting back..
Thes PK6 boxes arent unknown to have various faults usually mostly down to the internal bearings in most cases or lack of oil in the first place maybe due to aprevious job done on the van for example clutch..If all other 5 gears seems to be working ok inc reverse and your sure the clutch is performing as it should then it does sound internal im afraid..one way to maybe verfiy that would be to manually select sixth on the gearbox itself and slowly lift the clutch to see if it responds bear in mind it will most likly labor the engine or stall it anyway.unless you jack the van up off the ground so the wheels can spin and this will help see if 6th is still there.if theres still nothing then its most likly internal i would suspect.doing as above will or should help see wether this could be linkage problem if it does respond which i would advise first on checking anyway regardless..also maybe try draining out the gearbox fluid and see if that highlights anything in regards to internal wear as any glitter.mettalic swarf will be a sure sign of internal troubles..the only way forward as you'll know yourself if it turns out to be an internal problem is either strip down and rebuild at cost and transmission specialist - if you have locally you know off then it maybe worth having them verfiy further and would be the best place for that as they would be fully equiped to deal with that and or what most people do to try and save on that "the cost" would be to source another gearbox second hand provided it is fully checked over before hand..i do hope this helps.