I've just had a nasty thought that I was being daft - looking at the casing shape on the photograph kindly supplied by MBeige it looks a little bit like the left hand side of the differential casing in my photograph. So I have just checked my car again.

This bit of the differential does have a machined face but there are no numbers stamped on it.

I've also rather hopefully put the number I have found into the peachparts part finder - but got no joy. My trusty Haynes manual (sorry I hope that isn't a swear word here) has also proved itself to be nothing more than emergency toilet paper for help in this matter...