opening up fireplace for hole in the wall type gas fire

hi

i'm after a bit of advice, i'm going to get a hole in the wall gas fire fitted (by a gas safe engineer) but wanted to do a lot of the preparation myself to keep the costs down. the house was built in 1956. i've started to open it up and i think i've found the sides of the builders opening which as it turns out is almost the required width (see pic showing current lintel and a black line showing the height i want). however the height of the hole i currently have is about a foot too low. i realise i will need to fit a new concrete lintel and also dig out the concrete that was behind the fireback, but from looking at the attached photos is there anything else i need to be aware of?

i see that there's a couple of asbestos pipes either side of the opening can these be cut back flush with the sides of the opening and just filled with mortar - i'm guessing there may be a back boiler or something in there too?

behind and above the concrete lintel there's more concrete, do i just kepp hacking this away until i have the required height, and does it need to be made smooth or will the gather hood that comes with the fire be adequate.

if anyone can offer any advice it would be appreciated, thanks in advance.









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i've started to take out the course of bricks for the new lintel above the old lintel but i've noticed that the brickework either side of the flue is a solid 9" wall but only a single skin in front of the flue. how would i go about supporting the inner skin? if i was to put a 9" wide lintel accross the span it would partially block the flue which is obviously no good and i can't put shorter lintels in either side of the flue on the inner skin as there would be nowhere to rest the end of the lintel next to to flue. any ideas?