Cement screed on floorboards???????

[cd100]

Hi there
We are fitting a new kitchen, and laying ceramic tiles on the floor. The floorboards are uneven, and the floor is not level.
We have been advised to screed the floor with cement, and then tile on top of this.
My questions are as follows:
Should we lift the thin layers of hardboard which are covering the floorboards before we lay the cement screed (and if so, how do you stop the cement slipping through the boards?). Should we lay plastic sheeting down? I dont know what the cement will adhere to better. If we do leave the hardboard down, do we need to seal the floor, say with pva? How thick should the screed be, and how long before it can be tiled?
Sorry for so many questions, any help greatly appreciated guys.

 

[noseall]

do not under any circumstances lay a sand/cement screed on top of floorboards.

use 18mm w.b.p plywood instead, either by replacing the floorboards with said ply or going over the top of the existing floorboards if you have the height tolerance.