Levelling fireplace hearth and cement board use

[Rollin]

Having opened up a fireplace for the fitment of a stove, this is what it currently looks like.



You can see that there is a hole in the hearth that needs filling and the hearth as a whole needs levelling. A slate hearth will then be fitted on top.

What do I use to fill the hole (depth about 2 inches) and level up the hearth? If I should use cement, could anyone advise a correct mix and consistency?

I then will be lining the opening with cement board (Supalux). How do I fix the board to the brickwork? What do I use to seal where the boards meet in the corners?

I am aiming for it to end up looking like this, but with 3 larger pieces of slate forming the hearth.



Thanks