Pressure in Expansion Tank on Worcester 20/25?

Does anyone know what the pressure should be in my expansion tank on a Worcester Bosch Heatslave 20/25.
Problem:- Highest radiator filled up with loads of air, topped pressure on boiler and the same happened again and the pressure on the boiler went down a little bit. Pressure in boiler has now stabilised since last top up of pressure and doesn't seem to be blowing off. Running pressure at just below 0.75 bar. Could this be an expansion tank problem? If not what else could it be?
Thanks