Over Pressure

My son reported to me today that the pressure gauge on the central heating boiler (in his room) was low, even though it was still working, and that he couldn't hear any flow of water when he tried to repressurise it.

I went and had a look, the needle was on the border of the red, but the red at the high end, not the low pressure end. A check of the radiators didn't reveal any with air at the top, so I opened the bleed valve of the hallway radiator (with a bowl to catch the water). Why the hallway? - it has a wooden floor, not carpeted, so a drop of water wasn't going to do it any harm.