My squeak (or, rather, my 300Tdi's squeak) went for good after I had a "modified V belt" fitted. I can't remember what the LR part number is but I might still have the wrapper somewhere.
I had taken my Disco to a main dealer about 5 or 6 times to get this squeak sorted. Eventually I took it to a different dealer who suggested the modified belt. Apparently it's a softer rubber and is not suited to off-road work as it may get worn out if it's subjected to lots of mud/grit etc. The labour cost for replacing the belt was paid for under the LR warranty but I had to buy the belt mysalf as they can't replace one part with a different one.... or so they told me! It was about £12, I think.
I had this done about 12,000 miles ago (I think) and I've not heard a squeak since. This was after replacing the alternator, the belt, the pulleys and squirting everything with WD40, 3-in-1, talc etc etc.
I'll be having my 72,000 service (and cam belt change) done in the next month or two...... so am expecting loads of squeaks after that!
Judith