under tile floor prep advice needed please

[spreadandlay]

Hi,

i have screed floor, two different levels, in an old house which i will be putting marmox board down and UFH on then tiling, two thirds of the floor will have 20mm marmox fixed with addy or fixings and the other third has 50mm to make up before the 20mm marmox.

would you ;

screed up to height and run the 20mm marmox right through?

use 50mm celotex then 20mm marmox over that all mechanically fixed to original screed?

I would rather have the extra insulation under the floor just worried about the stability of it under tiles,

thanks, any ideas would be great