Size of Brick Opening for New Gas Fire

I've just removed an old back boiler from our lounge fireplace. To remove it I had to remove some extra bricks which had been added to reduce the size of the original hole. The actual brick opening is offset from the centre and had a row of filler bricks on one side to reduce (and even up) the hole so that it was central.
My question is whether it's necessary to reduce the size of the opening again for the installation of a gas fire and fire surround. The surround will actually hide the fact that the hole is large and offset so is it required for fixing purposes? The height of the existing opening had been reduced by resting a row of bricks on top of two old iron gas pipes which had been let in to the bricks on each side!
We are intending to buy one of those B&Q jobs where you can mix and match the surround, back piece and fire itself.
Do these sorts of fires fix to the surround itself or require support from the surrounding wall ? The sizes of the fires seem to be between 470mm and 520 mm which happens to work about right if I add a row of bricks to reduce the hole size.
I'll post a picture if it's difficult to envisage from the description above.
Can you get manuals for these types of fires online so that I can check the requirements before I purchase ?
PS. I'm aware of the testing required of the chimney itself and removal of the old metal flue used for the back boiler.
TIA
Andy.
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snipped-for-privacy@dsl.pipex.com wrote:

In our case, with a Valor fire and surround etc that you've likely seen in B&Q, the actual fire was held in place with tensioned wire attached to the back wall - this was pulled through holes in the front of the far, tightened to pull the fire back against the surround, and secured.

However, with regards to the actual size of the opening, the fitting instructions mandated a particular size, slightly bigger than the fire back itself and said that the rear opening must be bricked to avoid the potential for heat-related cracking of the surround. Do you follow what I mean? What I'm trying to say is, whilst the surround may have the perfect sized opening you can't just leave it at that - the back must also be bricked in.
I'm still unsure if that's clear or not... I know what I'm trying to say but having a hard time saying it!
Over to someone else with a better command of the English language...
Mathew
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Mathew Newton wrote:

closure plate & tape ?
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