We have removed a gasfire and the old wooden fireplace surround from a Victorian House. It looks as though when the original fireplace was removed, the original tiles were smashed out and the cast iron insert had been covered over with plain tiles. There is a lot of rubble (looks like it's been used as packing) and plaster supported by some chicken wire at the back. We intend putting in a new tiled cast iron insert and have a Gas Safe fitter fit a new gas fire. Do we need to be aware of anything building regs wise? Do we need to brick up properly where the rubble and chicken wire are? Or can we just board up ready for the the new cast iron tiled insert?