what you need to do jim is drain the pressure off the system until the gauge reads zero.

then get a good look at the top of the boiler for red round vessel with the car tyre valve.

with the drain for the CH still open pump the red vessel up to 1 bar.(the pressure gauge onthe boiler might move at first but just keep lettingthe water drain out until the vessel is at one and the gauge is at zero.)

if you are successful close the crain and top presure up to 1.5 bar on gauge.

turn heat on max and you should only see a slight movement maybe up to 2 bar on the gauge.

if this process doesnt work its time to call in the pros mate.