Stephen, sorry...bum advice...filling is via the filler plug on the back cover which is below the vent tube.. Sorry for the bum advice but because my diff is so difficult to get at I have a tube fitted where the filler plug is to help in changing the
oil.
The vent tube should be pointed upwards if fitted properly and should have a cap on it. The rear cover plate has internal baffles which prevent
oil from escaping from the tube but it should point up to prevent any minor
oil leaks from venting.
Cheers