OK.. Folks .. here's the update.
I chickened on doing the bearing change myself in case it all went to worms so I took it to my local garage run by a couple of old boys who are able to fix stuff without the aid of an OBD interface.
While removing the rear axle for the bearing change, a piece of differential came out with the axle. This pretty much explained the noise in the diff.
Salisbury 7HAs are difficult to get parts for, let alone to find someone here to overhaul them.
So the solution was to modify a differential from a Holden Commodore circa 1987 and at the same time convert to disc brakes. The advantages are easily obtainable (and reasonably priced!!) parts especially wheel bearings and seals etc.
This was done and it works perfectly, I also had the ratio changed to 3.08 (from 3.31). I am not sure where they sourced the disc brakes from but they had to be smaller and with smaller calipers to fit the old style rims.
Anyhow, the car is back on the road and drives beautifully - the ratio change is a big improvement as are the disc brakes. Maybe its my imagination but it seems to drive a whole lot smoother.
So many thanks to everyone who offered advice.
Thanks especially to Cheshire Dave who gave me a lot of his time on Skype.
