Cheers for all the comments guys
I actually worked at it last night and I can hear the c clip 'popping' as I tighten the diff - its coming out of its housing and when I loosen it pops back it. have tried widening it as far as possible, tightening in increments and same thing happens. I am not tightening too much just to the point where it moves by hand. If you move the diff by hand after about 30secs you can actually hear a small pop and suddenly the diff feels tighter but is slipping.
Have ordered new C clips and see if that helps, but will wait for the TD gear diff I think (unless a new c clip doesnt work).
Just out of interest, I seen these clips for DEx210 diffs guys - much cheaper than a tenner for 4 from some shops!! They appear to be identical.
http://www.ebay.co.uk/itm/STAINLESS-...#ht_500wt_1054