Bathroom tiling.. window sill question

Hi all!

I am retiling my bathroom, the first step of which was removing the old (and I mean OLD) tiles. Got them all off, but quite a bit of plaster has come off with 'em, revealing bare stone in some areas. I plan to use bonding to the bare wall, patch up with plaster, skim and seal with PVA.

Now the tricky bit is the window sill.. it's quite a deep job and was tiled up to the outer window frame, and I plan to tile in there the same. When the old tiles came off the majority of the plaster came out too, revealing a combination of breeze block and brick. I've cleared the rest of the plaster (there wasn't much!). There is also the wall cavity to consider, it's a good 3" or so. My question is this... what is the best way to cover the cavity to give a base for plastering/tiling? The builders or whoever tiled it last used slate to cover from the inside wall to the window frame, basically the full sill front to back. I was thinking marine ply (sealed), or maybe aquapanel? Then what, bonding and plaster to make up the depth (the tiles will need to reach the bottom of the inner window frame, it's a full recessed wooden effort), or possibly just plaster?

Any help appreciated,
Rich.

My advice is jst cover the sill with some plaster board dadbed on then tile over it.

What he said.
There's nothing to stop you using a few layers of plasterboard if you need extra height (just stick em together).

Cheers fellas, that makes sense. Much appreciated.

Rich.