Dodgy diff

[Drifter]

Have a 2000 1.6 Vauxhall with only 6k miles but have been hearing a slight whine in recent weeks. Thought nothing off it until 2 days ago when the slight while became an almighty one - especially notable at constant speeds on a light throttle. Drones out the engine at 30mph!! Spoke to Caterham and incredibly helpful specialist and both said the diff.

 

Had a look and it seems that the opening where you put oil in has no cover ie. there is an open screw thread. Don't know how this can come adrift as was told it is an allen key job - is it by any chance part of servicing?? Anyway, anyone know what diff oil to use and where I can get a plug to put back on? Just hoping diff is still OK as have driven about 150 miles since the whine. Was the only car available and what weather....!!