Putting range cooker inside a fireplace

I've bought myself a Rangemaster Classic 90 to put into the fireplace in the kitchen. Said cooker is exactly 900mm wide (I measured it!), but the fireplace is 890mm wide. (I knew this before ordering). I've got a brickie coming in to modify this (and make the hole extend upwards a couple of feet more).
I need to know how much wider than the cooker the finished hole needs to be. Do they need a couple of inches to move them into place, or is this unnecessary? Is there a gas regulation limit to the height of the opening above the hob? The opening will be non-flammable plaster on brick.
Christian.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Supposed to be 760 mm from hob to surface above: you'll probably need something like this anyway for it to be usable
-- John Stumbles -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -+
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.