Thanks for your reply GolfyG, much appreciated. That's incredible Jag wanted over £1200 to replace the seal! Looking at the service history of mine, the last owner paid about £350 for Jag to change the seal back in 2018 25,000 miles ago. I was aware of this before I bought the car but I was hoping that work resolved it back then. It certainly went fine for 4 years and 25,000 miles with no problems. When I took the car in for a service on Friday I was advised there is a noise from the back and that the seal is moist. I can hear a very, very light noise very much the same as an air con fan and I can only hear it as a passenger.
If it's already making a noise though it's likely that the diff is damaged and I'll need to fit a used one. I'm just not sure whether to get the diff oil changed and a new seal put on or whether to just go for a new diff.
I can get a used diff quite cheap on eBay and I'm told it's 4-5 hours to fit so although I could do without this, it's not the end of the world price wise. I've heard stories online of people paying several thousand for a diff but it's hardly the same as having to change an engine.
Ideally, I'm hoping to keep the car for many years so if this is sorted, hopefully it will run fine. It's probably tempting fate lol but at the moment everything else apart from the diff is fine, even the main dealer health check didn't find any other faults.
Can I just ask if changing the oil and seal cured yours and what type of oil you used? I've read that the following oil is recommended:
Castrol TRANSMAX Axle Long Life 75W-90 Fully Synthetic Rear Axle Fluid
https://www.opieoils.co.uk/p-737-castrol-transmax-axle-long-life-75w-90-fully-synthetic-rear-axle-fluid.aspx
Also, I've seen brand new 2.2 litre diffs for £575 on eBay but I'm not sure if these will fit my 3.0 XFS - off the top of my head, I'm sure all XF models apart from the XFR with the e-diff used the same diff but I'll have to check part numbers first.
Jag didn't give any verdict on the diff, they just noted the noise and that the seal was moist - I haven't noticed any drips on the drive so I guess it is possible it hasn't lost much fluid but Jag wanted £132 just to have a look at it and my logic is Jag aren't used to working on diffs very often and even if they did a good job they'd charge a fortune! So whatever happens, I think I'm best staying away from them for this work. They promised me a video showing the moisture but I haven't got one - I'm not sure how easy it is to see the seal on the driveway.
Thanks again