Danny, ok, that's good to know about the studs being on the alternator, not the ring. And also about cutting the ring diameter - would that explain the lack of part number on the ring? I presume so, but if mine was reworked to reduce the diameter, they did it major league style - there are no ridges, grooves, part numbers or anything on this baby! It's like a CIA wetworks gun - no traceable markings. ;-)
So, I went out to the garage and took these pics. What they prove to me are:
a) quite apart from the lack of part number, the only distinguishing mark on the ring is the extra depth of the ring around 1/4 of the circumference on the right of this pic.
and
b) I can't get the existing alternator out of the casing! There are 4 long bolts that attach from the 'front' of the casing to the alternator (below) but if I undo these it's still wedged in firmly. There are three threaded holes in the centre of the assembly below which look to have bolts screwed in from inside, but I've no idea how to take this further..
Looking at the Valeo alternator, there is a Valeo option here in the UK (£260 + delivery) as well as Mahle (£166) and some unknown brand for £71! Worth investigating. But first I'm going to measure the ID of the space the alternator fits in. I've already bought something without doing enough homework, so don't want to repeat the event!