Dan

The sandy stony stuff sounds like sand/cement render. It's what's on the brickwork before plaster.

Assuming your tiling it when you've made good....

If the depth is enough I'd 'dot n dab' a little plasterboard on it (you can get thin plasterboard) or even use some of the old tiles to tile it up to the right thickness and then tile over the lot.

If you're plastering it all to paint or wallpaper it or whatever then you can just plaster over the render (get the right stuff though, some is just skimming plaster these days).