Recently I have removed a pine fire surround with cast iron back panel from my fireplace, as it was a solid fuel fire my intention was to have a wood burning stove in the gap. The gap is 21 inches across and the chimney breast is 72 inches across. I want to increase the size of the gap. A fireplace trader informed me that I can increase the gap to 40 inches, however another has advised me to make the gap no wider than 30 inches as it could undermine the chimney breast. Who is right?. Does the work I intend to do come under building regualtions or can I just make the gap bigger. (I do intend to get an expert to do the work). Any advice appreciated. 