Interesting problem that we had but with wooden floorboards. Went to Google search engine. Can see examples of levelling compounds that do up to 120mm. Of course the constituents change from latex based to cement based.
When 'professionals' did our kitchen they did not level. Just screwed 9in ply over old boards. As the ply was under tension over uneven floor within weeks several tiles worked loose. Removal done by smashing them. Aaagh!
[Edited by Hamiltonandy on 06-11-2002 at 21:29 PM GMT]