Well, I've swopped the diffs - grateful thanks to all who listened and contributed patiently to my endless questions.
The only problems encountered, apart from the stupid lop sided weight of the item - I've no idea how my wife carried it into her car but apparently I owe her a lot of favours now - were getting it off the studs and manoeuvring it around the bottom of the self levelling gas strut. I thought I had it all nicely set up for working on with the axle on high-ish stands only to discover that I had to lift the body a few inches to raise the bottom of the strut. Thanks heavens for high lift jacks.
Studying the old diff didn't reveal any missing or broken parts; just more play in the bearing than the new one has. However, the differential cross shaft sticks out at one side and one of the bearing nut locking fingers is bent. Would either of those matter? There are pictures here:
Richard