This should be a pretty straightforward visual of the fireplace and chimney breast going in place.
I used double side sticky tape on the bottom of the chimney breast. The thinking being that glue would make a mess if it dragged on the floor. Ideally it wouldn't touch the floor as there was enough wriggle room to get it in without doing that.
My usual De Luxe model glue on the back of the chimney breast and fire box. I find it sticks to paint better than wood glue does and the wall it was going to was painted.
Remembe the previously drilled holes and the cocktail stick marker, that was I was working with to get the 'unit' in exactly the right place. It was all a bit fiddly locating the hole in the fire back without shoving glue hither and yon. Success though and the chimney breast shoved nicely on to the back walls, no gaps, no funny angles. I do like this MDF kit.
The marble hearth in front of the fire measured six inches the chimney breast measured five so I used an easy guide of a six inch ruler to glue the hearth down at the half inch mark.
Everything nicely centered and the grate and coals could go in. This isn't glued in.... just in case?
The mantel went in in place just by eye as there was very little room for error
I now have a room just waiting for all its wood trims. Not especially looking forward to a couple of days painting trims but needs must .......
Major learn from this ..... no need to get fancy schmantzy and drill the hole for the fire wire at an early stage and then spend ages fiddling a 'unit' into place. Skip that move and just measure and mark where your chimney breast will go (on the wall or floor using masking tape) and glue it in and, then drill the hole for the fire wire through the fire-back and back wall. Obvious really.