Gas Fire Installation, hole in the wall?

[PrenticeBoyofDerry]

Good Morning,
I have now had old gas fire removed from chimney breast.
The new fire has arrived, it's valor harmony just for the record.
The hole size left after removing the old fire, is somewhat bigger than the new one requires.
Current hole is 650mm wide x 675mm high and 400mm deep.
The required hole is 457-410mm x 565mm x 225mm aprox
My question is I intend to line out the hole using precast concrete slabs, so the sides and the back are lined the slabs will be 100mm thick and jointed using cement mortar.
So is this okay to do and what is best to do with the area below the fire as the fire will not sit on the hearth but be above by about 100mm, the area around the fire will be fire boarded and plaster finished.
I am fitting a hearth, its the void underneath the fire should that also be filled in.

 

[kirkgas]

if the fire is to be wall mounted you simply fill in the void at the bottom as you are going to do the sides and back, take care to measure carefully to ensure your void ends up the right volume and you will be fine

 

[PrenticeBoyofDerry]

By the right volume?
The installers/users manual gives a range of measurements.
i.e. the width is 457-410mm (off the top of my head), is it better to be in the middle of these parameters to top side or to the small side?
and does the depth need to be exact or is that not a factor proving it is deep enough?

 

[kevplumb]

give or take a kick in the ass

as long as the flue works ok

that is the important bit

unless yer want err indoors sparked when she falls asleep drunk

and don't kid yourself

we have all thought about it

 

[namsag]

Make them to the small end of sizes, gets a better seal

 

[PrenticeBoyofDerry]

unless yer want err indoors sparked when she falls asleep drunk

and don't kid yourself

we have all thought about it

So I'm not the only one thinking about doing my missus in

I've had the flu swept, the sweeps seems to think it's in good nick. But I'll have the guy who's fitting the fire, check the flue first.
Looking on the negative side if the flue failed what are my options, is the only option a liner, if that is an option?