Do I need a non combustable hearth?

If I do not need a hearth, I will install my gas fire. I'm looking at the installation instructions for an older Potterton gas fire which covers models Allura, Brava Charm and Decor. I believe these instructions are made in light of The Gas Safety (Installation and Use) Regulations 1994 (As Amended).

I wish to install the fire wall mounted. At 2.7 it says

" 2.7 Wall Mounting

If the fire is to be wall mounted over a combustable hearth or floor, the bottom of the fire casing MUST be at least 100mm above the level of the hearth or floor covering. "

Now, unless things have changed in this area, I believe that I can lay carpet below the fire, as long as the fire casing is 100mm or more above the carpet. And that this is within current regulations. And that I DO NOT need a non-combustible hearth.

Can anyone please confirm this? Thanks.

P.S. I am designing the brickwork, I will have a gas-qualified person fit the fire.

Actually, my best bet is to ask Potterton, see what they say.

I have another question: Originally, the fireplace and flue arrangments were built for an open coke burning fire. I took out the contraption that was in the firebox and it's now just an open space. The original bricklayer coated the inside of the firebox with something . It's harder than plaster. I think my mate said it was something called browning.

I'm adjusting the size of the brickwork that makes up the firebox and will now only run a gas fire, not an open fire.

What concerns me is that I get the sealing right. I want to ensure that no flue gases leak.They should not get through the brickwork, but would it be customary, the norm, to coat the inside of the firebox with something? If so, with what? Just cement? Or something specially for the job. Thanks.

P.S. Understand that my old gas fire I'll bolt to the wall. And behind it will be a cavity formed by the firebox, which has an opening into the chimney stack.