I have a Glow worm swift flow, the problem is when the boiler is off, the
pressure drops quite dramtically there is NO visual evidence of any leaks,
then when the boiler is switched on the pressure can in crease to as much as
3bar, one radiator upstairs never comes on, despite being bled and water
coming out, both pipes never to radiator never get warm.
What do you think.

Not a plumber etc.

Sounds like the pressure vessel is knackered.