Gas meter box build in

[davidmiller]

Was wondering if anyone knows anything about how these need to be put in. I'll attach a picture that SGN gave out but I'm not sure about it.
Firstly they seem to be saying that a membrane above should be 1300mm wider than the box. So that would be nearly 1700 wide and unless I'm mistaken that goes behind the box which would mean no wall ties possible in the brickwork. Then it seems to be tucked under the box itself but no other company that I know requires this.
Can anyone make sense of it please?

 

[vincentianpinner]

Where I used to live that sheet had to be 700mm wide. So maybe 1300mm should be 130mm but times 2 as there would be one at each side.

 

[davidmiller]

Where I used to live that sheet had to be 700mm wide. So maybe 1300mm should be 130mm but times 2 as there would be one at each side.

Makes sense. Ta