Can I put a natural tile on the wall instead of the wall screens? Does this comply with the 'non combustible wall' requirements?

Any non-combustible mineral board that is directly fixed to a combustible material like timber immediately loses its non-combustible qualities and therefore the full clearances must be applied. If this board were to be packed 25mm off the combustible material underneath and then ventilated top and bottom with 25mm air gaps then this would become a non-combustible wall covering. This can be covered with any other non-combustible material you like such as tiles etc.

As a note of caution please be aware that Fireline Gib is not a non-combustible wall covering.