You could try these guys
http://www.transend.co.uk/newparts.htm or
http://www.berlyn-services.co.uk/specials.htm but Bert had none available earlier this year. When I replaced mine I put the bottle I was using to fill with the oil in a bucket of very hot water to get it runny enough to be able to squeeze into the Diff, it is very thick otherwise and like mentioned earlier make sure you can get the filling plug out before you drain the Diff, from memory 17mm Hex, I bought one with a 1/2" socket drive as access on an S2 is awkward without dropping the exhaust shield however with a long enough tube and squeezy bottle I managed OK.