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My boiler keeps losing pressure. I have noticed the is a water mark on kitchen ceiling, is this why the boiler pressure keeps dropping, it was not there before they re pressured the boiler

We had gas and electrical work done and are now left with a missing patch of floorboard in our airing cupboard. The size is roughly 40x25cm where the floorboard was cut out. We still have the cut pieces but there isn't anything to attach them to. The original floorboard plank runs under a plasterboard wall. How can we fix/replace the floorboard? Will the wall need to come down? They are planed timber not tongue and groove type.

Does a fuse box have to be on an outside wall