Some developments. I've contacted my local Toyota dealer and the 2 dealers who had previously serviced the car and all were very helpful. It turns out that the car has only ever had "interim" services, so the diff oil has never been changed. Interestingly 1 of the dealers said they'd never had a problem with diffs (although I don't suppose they get many owners of new cars who ignore the servicing of them completely!) and another said they'd only expect to service them at 30-50k miles. I took that at the time to mean they'd only change the oil then but could they have meant something else? Is there anything else to do on them at that sort of mileage?
Anyway, we've gone back to the seller and said we'll go ahead if he pays for a full service, including attending to anything missed out on previous services and not covered by the 10 yr service, and the car is given a clean bill of health particularly with regard to the diff. That would have the added benefit of getting a Toyota warranty from the date of the service till the car's 10th birthday-only a few months but useful nonetheless. The seller is thinking about it! I'll let you know what happens, but in the meantime thank-you for your contributions.
keith