Ok I must have the F17 as reverse is as you said.need to get under with the cover off and have a good lookThat won't work, because you won't get into the bottom of the diff housing.
If your box is an F17 (reverse gear the stick goes over to the left & forward), it has not got a drain plug, and you need to remove the diff cover plate to get the oil out.
To refill, there is a level plug on the nearside of the diff housing. you remove this, and put oil in through the top breather plug until the oil runs out of the level plug.
If it is the other type of 5 speed box (M32?), that has a drain plug & a filler plug on the back of the diff housing. You refill as with the F17, through a plug on top of the gearbox until the oil runs out of the level plug.