@ RogerM,
I dismantled and rebuilt the diff last weekend and used non kit grease. I think the main noise is coming from the spur gear (seems to run a little off centre. When I tried to tighten the diff I had lots of screw movement so I doubt that sanding back the nut holder by 0.5mm has been detrimental. I'll bring the buggy tonight and if you get the chance you can have a go at setting the diff tension for me.
Cheers
Si