its a 12 mm allen key. filler is next to reverse switch facing forward,make sure you can undo this before you drain the oil,also check the level before draining,the drain plug,also 12mm,is the lowest plug under the drive shaft on the passenger side.............................[/quote]
Thanks multipete,
I kind of jumped the gun; an engineer friend of mine checked under the car and identified 2 plugs. One, as you confirm, is the one I thought it was alongside the reversing switch. Not too sure about the 12mm drain plug; I undid a 10mm allen key plug at the lowest point under the drive shaft and out poured the oil. It's a neat fit; to gain acces I had to use a short crank allen key. I didn't remove the fill plug first, but I slackened it off. As to quantity of oil, the box took about one and three quarter litres before backflowing out of the fill plug, so allowing for the remains of the old oil clinging to the gearbox internals, hopefully all should be well.
One thing that surprised me was both plugs appear to be taper fit; I guess the clue is to tighten them until they feel right and not overtighten..........