Fireplace l

Apply silicones or caulkers for every small gaps (a few millimetres), a high temperature silicone of flexible filler might be fine

Frame it with a Custom Trim or Flange (Best for small gaps) If the gap is relatively small (less than 50mm), you can create a decorative frame or "flange" to hide the edges. Aluminum or Steel Flat Bar: You can buy flat metal strips, spray paint them to match the fireplace or the wall (often matte black), and fix them around the perimeter of the fire. UPVC Trim: If your fireplace is white or has a specific modern finish, a simple plastic quadrant or flat trim can be glued to the edge of the fireplace to "extend" its reach to the plasterboard.

I specialise in media walls and this is what I would do to make a seamless invisible modification. You need to follow the existing timber framework and add plasterboard on top of that. 1) Install 3 strips of wood on the top and 2 sides, set it back 14mm. Make sure the new fireplace overlaps the new opening by 5-10mm. If you're unable to rip cut the wood, you can buy strips of wood at Wickes to suit. 2) Add matching strips of plasterboard (12.5mm) on top of the wood, glue in place with CT1 or other adhesive. 3) Place filler over the new plasterboard and level with the existing plasterboard. Sand and feather in. Then it's ready for paint.

The best solution depends on the size of the gap and the type of fireplace. Usually the opening can be adjusted with suitable framing and plasterboard, then finished and decorated so the fireplace looks properly fitted rather than simply covering the gap with trim. However, it’s important to follow the fireplace manufacturer’s required clearances and ventilation instructions, especially if it produces heat. I would check the fireplace specifications and measurements before altering the media wall.