I am preparing the space for a hole in the wall type fire in readiness for installation by a gas fitter. I have made a cardboard box to the exact dimensions given by the manufacturer for the hole size required. This can be seen fitted in the existing fire hole in the photos. The box beneath is just to hold the upper box in position. I would like some advice on the best way to build out the sides and top, (the bottom is not a problem. The left side gap is around 10cm so I could use bricks on their sides. The right gap will be trickier as at a maximum width of 7cm, it will not take a normal brick. The top gap is 3cm and needs to be filled to about the depth of one brick. How is this best done? Is cement the best material to use here? This gap cannot be reduced by raising the fire, due to some upper brickwork preventing this. The fire has a 40mm flange around it which is screwed to the surround onto a foam seal. Thus the area around the hole needs to take wallplugs 20mm out from the hole. Therefore the hole needs to be fairly snug. Any advice welcome.
