Mail order from Opie oils.
It's the only place I've ever seen it, Jim.
I think the Shell website advertises it but I haven't seen it anywhere.
The Opie stuff ain't cheap, either. I presume you will agree that you will need two bottles for a complete gearbox oil change?? I did because I thought they were one litre bottles. But they are not, they are 950ml, so two isn't quite enough. So I ordered another which arrived today just to use to top up a little bit and then read on here (in the Bible section) that the box requires 900ml. So I needn't have bothered buying that third bottle after all.