Pressure vessel not set to the mains pressure
You can try removing the dust cap and depressing the shrader valve for a mil second eg release some air and see if it changes etc
That’s why you over-pressurise it slightly eg 1.3 bar ends up being around 1.1-1.2 bar when you remove it
Also did you drain the boiler and leave an open end while re pressurising the expansion vessel
Also you should be gas safe registered as your inside the combustion chamber
It’s not just a pump you will require a cold water tank as well eg all in one unit dab Easy sys eg
https://www.anglianpumping.com/product/water-boosting-pumps/dab-e-sybox-with-e-sytank/