Bleed off some water from a rad so that pressure drops below red when hot. Check pressure when cold, write figure down.
Check it again when cold 24 hours later - it should be the same. If it has gone up, then water is trickling in via the fill loop as Polar bear described.
If cold pressure is constant but when hot it is going into the red, then either the pressure vessel isn't working, or air is getting into the system - both could be due to pressure vessel failing.
I did this yesterday, bleeding off the rad in the hallway (as I can actually see the gauge while I do this - this is the only rad, btw, that I cannot fully rid of cold spots).
I reduced the pressure to the top end of the green area / bottom end of the black on the dial.
The upshot of this was that by this morning, the pressure had dropped sufficiently low that the boiler wouldn't ignite to give me hot water - never mind heating.
This evening I restored the pressure and ran the CH.
After about 10 minutes the pressure gauge had gone into the red, so I switched off.
Gary, the heating engineer has been summoned.
I hope it doesn't get too cold between now and Nov 13th when Gary is due to call.