Diff lock linkage adjustment.

Hi, Does anyone know how to adjust the diff lock linkage on a 300 tdi. If I engage the diff lock lever I get no light on dash in it does not engage. if I crawl underneath and lever the arm over slightly on top on the transfer box the light comes on and it engages. I've been told there is an adjustment for this but I can't see any reference to it in the workshop manual. Any advice would be greatly appreciated.

if you lay under the car to the front and look back over the top of the transfer case with a good light you can just see and feel the linkage adjuster, it's a little fiddly, sorry can't remember what size the nuts were though could also get someone to move the shifter to make it easier to locate