Expansion vessel pressure reading different to CH pressure?

[Scribbler]

Why is the air pressure in an expansion vessel at 0.8bar when the CH water pressure is at 2bar? I ask as I have had problems with the prv leaking so this was replaced but I noted CH pressure exceeded 2.65bar when hot and new prv started weeping - grrrr! Confirmed ch water capacity to be about 80lt and less than limit for boiler. Then thought expansion vessel not working so did some tests. I have unpressurised the expansion vessel and CH then undid boiler connection. On repressurising the vessel I saw no water come out of its feed so think diaphragm is intact. Also air pressure seemed to hold at 1.5bar. Reconnected the expansion vessel and reduced air pressure to 0.75bar as standard on Halsted High Ace combi before charging CH to 0.7bar. On running CH I find pressure of CH on gauge rises more than expansion vessel air pressure. Any ideas?

On another matter - replaced noisy fan at cost of £180. Now find it is so easy to replace bearings at a cost of £6.50 the pair - if only I had known!