200 Tdi Discovery transfer box problem

Having problems with my old 200 tdi T/F box..
The vehicle drives fine in high box but will not engage low range. The transfer lever moves a little to freely (more than it used to) and slops about where the low range used to be, it does move back into high range easily and the difflock works in high range.
I've been underneath and the linkage to the box is OK and I'm unsure what to do next.... I'd like to drop the T/f box repair or replace what ever is broken and put it back togather straight away as I need it every day but am not certain what else the problem might be.....
If anyone can shed any light on the problem I would be most grateful..

on the t/box were the linkage joins ,remove the 6 bolts holding housing on and check grub screw on shaft on the under side

I'll do that first thing tommorow