I bought a fire surround a year ago and have still not installed it... The reason is that I ordered an 'off the shelf product', assuming that it would just fit flat against the wall (apart from space to add slips). It doesnt fit flat, there is what i believe is called a 'return' which means that the outsides of the surround are flat against the wall but the insides have a big gap between the wall and the surround.

Does anyone have any ideas about what to do about the gap?! I hate looking at my fireplace at the moment, desparate to get it sorted! we live in a rural area so although it is possible to get someone out to look at it, the costs of doing so are pretty ridiculous so id like to avoid that if possible and my partner is capable, just in need of direction!

ive added picture of the space, another with the fire surround held over it and marked to show the gap and finally a pic of the sort of thing i would like to end up with.