What screed can be installed ~30mm deep on a floating floor?

[ToastyToes]

I have ~55mm I need to fill to make the conservatory level with the rest of the house.

I would like to lay 25mm Celotex to help reduce how cold the floor is before I lay underfloor heating. I know 25mm Celotex isn't much, but I'm sure it will help.

Sand/Cement floating screed needs to be a minimum 65mm for regs. Is there any other type of screed I can use in this scenario?

P.S this is an old conservatory we are making better as part of our renovations. It has a 100mm polystyrene layer below the concrete.

EDIT: Does the concrete base need to be level before laying the floating insulation? Some areas are slightly raised from removing the old screed. Only a mm or so.

Thanks.

 

[noseall]

Use thicker celotex and 18mm ply.

 

[ToastyToes]

Use thicker celotex and 18mm ply.

I guess i'd need to use some self leveller on the concrete to get it level before laying the celotex and ply otherwise it'd be impossible to get it level with the rest of the house.