Thank you for your replies, it has helped our mental state considerably.
Yes we hit the roof too after the second diff, but what we were told was it was a seperate fault and it was a faulty part that was fitted. We had the dealer look it over this week and they have not found any fault, so I will keep my ears open for any further noise from the back end. The first failure manifested itself as a rumbling noise from the rear of the car when at 70Mph in sixth gear then planting your foot. I perhaps was overly sensitive thinking I was hearing the same noise again ths week, but as I have winter tyres on PERHAPS it was that, not entirely convinced but will wait and see. If it fails yet again I will be going back to the dealership with a view to giving them back the car and recovering my money, which would be a shame as they have been good to us and we love the car.
So as it stands at the moment I am waiting for it to either fail on me, or not and will be happy ever after. LOL If anything changes I will ask for more advice, but I hope it will be smooth sailing from now.
Si and Marcia