Thank you for the replies, I can now advise how I have got on. In Australia we have many fewer old spare parts, all my inquiries here for the required flange were fruitless.
I have had the too large original flange modified as follows.
1. the holes for the bolts were filled with weld
2. The diameter of the flange was reduced to the new smaller size.
3. New holes were drilled, between the old ones.
4. An insert was machined and shrunk in, to take the locating flange from the driveshaft.
I now have installed the flange in the rear of the O/D unit together with a new seal.

I was fortunate that the bright surface against the seal was in good condition,
probably very fortunate as the car had sat beneath a banana tree for 10 years!!

I am blown away by the knowledge available in the group. and your readiness to share it.

Again thank you for your replies