What I would do (and have done in the past) is find the highest point and use that as your datum. Then run 3 screeds made up of floor tile adhesive and old tiles or slates or any old bits of "quarries," make them about 3 or 4 inches wide. You can get the rapid set adhesive for this, dont use the expensive stuff (Blimey Denmark have just scored
) but use only powdered stuff. Make sure your screeds are(bloody Hell England have equalised
) spot on. Then fill the gaps inbetween with adhesive, tiles, slate, gravel and anything like this to bring it up to screed level. Make sure you "bed " everything into the adhesive (dont make it too runny just firm enough not to leave voids).
Like I said I have used this method on floors that are going to get tiled as I have never used self-leveller, this is a good method if the depth to fill is not to deep.
Ps Any old wall or floor tiles I use to "fill" in I always put the glazed side down, it might not make any difference but when you come to tile you are putting your adhesive on the right side of the tiles.