You may find that the 'constructional hearth' is still underneath your laminate. I think for fire regs that you wouldn't be allowed to put ply on top and then tile, you'd have to use concrete or similar to level it and then tile.
Also that opening in the chimney breast is tiny - I expect you've got a much larger 'builder's opening' a bit higher up. If you're not too precious about the plaster I'd advise a little exploration with a cold chisel.
You may find that the 'constructional hearth' is still underneath your laminate. I think for fire regs that you wouldn't be allowed to put ply on top and then tile, you'd have to use concrete or similar to level it and then tile.
Also that opening in the chimney breast is tiny - I expect you've got a much larger 'builder's opening' a bit higher up. If you're not too precious about the plaster I'd advise a little exploration with a cold chisel.