there is no drain plug, but you can unscrew the bolts and seperate the back plate from the casing with a screw driver and i will re-seal. Or you could just take the diff out, but thats a big job just to change some oil.
I think it takes about 1L of 80w gear oil. But fiill it up till it comes out the fill plug.
Also before tring to lossen the bolts give them a "tap" they are always very tight and can sheer.
Bruce