Cost associated please

As DrRob says LR describe them as "sealed for life", but Q what they regard as "life" (they'd like you to scrap your car and buy another rather than keep going...). I had thought the 8 speed gearbox manufacturer ZF envisaged about 75,000 miiles, but it does depend on what sort of driving your car has done, so lots of heavy towing in hilly terrain, hard use in a really hot country etc will mean the mileage would be shorter. I think some of the independent technicians on here regard 60,000 miles as best practice (happy to be put right on that).

Not having needed a change yet I cannot comment on cost, sorry. Only Range Rovers since 1988

If you can buy the fluid and filter the LR book rate for doing this used to be quite reasonable. It was a few years back but iirc I paid hunters in Derby about £100 in labour.

Best to source the filter and fluid rather than buy off a garage. There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
KIA E-Niro 4+
2021 Discovery 5 D300 MHEV commercial

I actually just did mine a short while ago 4.4 SDV8. Its a plastic integrated sump and filter, transfer oil, transmission oil (life guard 8 ZF) and front and rear diff oil. I used a large syringe to replace the fluid.