Hi Sebastian. Have you had a price quote from a dealer to do the job? You should probably be sitting down when they tell you their price to do it.
Here is a link on the RR.net
http://www.rangerovers.net/repairdetails/headliner.html so that you can see what is involved in the job. You can decide if you want to do the whole job yourself or just remove the liner shell and get an auto upholsterer to put a new lining on - you just remove and reinstall. Just get a mate to help and you provide the beers.
If you decide to have a bash yourself the lining material should be readily available at any auto upholstery supply shop - they will know exactly what you are talking about and also have the glue etc. Just ask around and thay may need to see a small sample of your existing lining to get a colour match.
Mine is sagging at the rear and so that is another job to do.