If you are planning on keeping it a while, worth doing gearbox oil at that age too. Manufacturers don't put gearbox oil in their service schedule, but it really isn't a "life long" fluid, unless you want the car life to equal the gearbox life...
It is indeed the diff - fuck! Good diagnostics TT. So on the face of it I have two options I think. Option 1: drive the car until it goes bang, then scrap it. Option 2: source used diff online and get it fitted. I can find something that looks like it would fit on ebay (https://www.ebay.co.uk/itm/226120027494?chn=ps&_ul=GB&norover=1&mkevt=1&mkrid=710-166974-033325-9&mkcid=2&mkscid=101&itemid=226120027494&targetid=2276391439863&device=m&mktype=pla_with_promotion&googleloc=9046603&poi=&campaignid=21069693970&mkgroupid=165332018528&rlsatarget=pla-2276391439863&abcId=9387975&merchantid=114897452&gad_source=1&gclid=CjwKCAjwl4yyBhAgEiwADSEjeIAhYaVfOVQxJyVyRVZPn13pZ-iK6NI2Qia0YqajrHqIWlAL7EvtwBoC5ToQAvD_BwE) but it does rather look like it's been salvaged from the Titanic. The mechanic I spoke to about this was dubious it was worth it due to the general rust below the car, but I am quite unwilling to commit to car searching again, especially after it just passed it's MOT...Anyone know roughly how much labour is involved in replacing a rear diff? I'll obviously ask the mechanic as well (who is very good and I am confident he won't rip me off).To top it off while it was on the ramp the mechanic pointed out a lump in a tyre, so I've got to get one of them as well...
Sounds like a result.