Worcester 28CDI Boiler Pressure?

Help !
The Boiler was accidentally overfilled to 3 Bar.
The inlet valve is shut off but every time I bleed a radiator the pressure drops to around 2.5bar but then instantly rises again to 3bar as soon as I stop bleeding it. It's as if it's directly connected to the mains. There is slight drip from the overflow but not much.

How do I drop the pressure back to 1bar ?

Thanks in advance for any help.