fitting new fireplace and gas fire

Hi,

I have just fitted a white micro marble mantlepiece and hearth to an existing bricked chimney breast, and the corgi man is booked in for tomorrow to fit the gas fire.
The whole lot is a kit from B&Q, with the back panel already attached to the mantle piece.

My question is....my corgi man mentioned on the phone "have i sealed the mantle piece to the wall"? I said that its just screwed to the wall while sitting on the hearth and then the new gas fire fits into the hole in the back panel.
He said again that it needs to be sealed to the wall either using rock wool or a fireproof sealant.
Is this normal practice as it doesn't mention this in any DIY guide that i have been reading anywhere?!?!?!?!?

Surely the gas fire shouldn't release unburnt gas that could escape into the room......however if that is what i need to do then i need to do it before tomorrow afternoon.

Any comments please !

Get the corgi guy to seal it when he comes around.

Thanks, i asked the corgi man for his advice when he came round, and he actually gave me some rock wool to fill the gaps and then just got me to plaster the remaining gaps from the inside.....so its now all done....hoorah