F.Y.I.
Differential replaced today in 2 1/2hrs @ £50/hr - no more clonks & clunks, appears to be a job well done. Not sure if the mechanic took the easiest route, but probably the shortest, by removing all drive and prop-shaft, Front & Rear Mounting plate bolts and finally manipulating the 30 kilo diff out of the Sub-frame.
The trick now is to be patient and meticulous with the running in period of 1500 miles with 1000mile oil drain-off and replenish with, I think, the Morris Lube Lodexol XFS 80w -140. And then continue with bed-in process for the next 500miles but with careful and varying amounts of speed and acceleration.

If any you are thinking of taking this on in the near future it?s not as daunting as you think and you don?t want to pay a certain OEM price from a well known part supplier, there is an alternative one, but I?m not sure how many units are still available, probably very few as these were on eBay recently.
Contact details as below:

Mark (Manager)
Mark (Parts Person)
South Lakeland Mini Centre
Barrow-in Furness
Tel: 01229 836638
mark@southlakesminis.com

If they want to know how you contacted them just say Phil bought one recently and gave me your details ? its ok guys, I?m not commission here.

Anyways, thanks to all for your advise/experiences/contribution ? every little helps.

Phil.