I'm fitting a new limestone mantle and cast insert and looking for advice on how to fit it. I'm not doing any gas work, this will be a gas safe plumber.

When installing the limestone surround should I take the plaster off and fit it directly to the bricks or can I fit direct on the plaster?

The cast insert... Is this simply put in place and held in by the fire surround or is this fixed in some way? Does the cast insert require fixing to the surround or does it just butt up together

Any other tips would be great.

Thanks in advance....