Hi Ian
Really sorry to hear this. You are getting more than your fair share of trouble with this, they are usually more reliable.
You may be well versed in this, but a couple of things to look out for;
One, check on consumer law and statutory rights. If the diff is new and guaranteed, you as a consumer must be looked after by the supplying garage in Exeter. That's the way it is here, I think English law is the same. If the unit is faulty from the manufacturer, then the garage have to chase Swindon Subaru, not you.
Two, don't let yourself get sandwiched between them as they fight about who is liable. The Gearbox centre put in a diff that did not work. It's up to them to fix the problem for you. If they start stretching out the off-road time saying they are waiting on a decision/inspection/engineer's report/legal advice or any other guff, tell them you will sue them for use of a hired vehicle while they are dragging their heels on fixing your car.
You will find if you ring the Consumer Association, I am reasonably correct on the above. If you let them drag it out, they will, as neither of those parties are going to want to foot the bill.
Best of luck with it.
Joe
PS Why don't you come over to JAE on the Saturday 24th, and have a chat about it with other owners?