GlowWorm Swift flow 80 combi boiler-pressure too high???

Can anyone help me as I am paranoid my boiler is going to blow up every time I put the heating on, so I am now not putting the heating on and it is freezing!! Last week we bled our radiators as they were all very cold at the top, and the pressure on the bolier went right down to about 0. We had a plumber in the flat at the time and he managed to adjust the pressure, although I think increased it a bit far, so it was at about 3.5 bar. He suggested we needed to let some more water out of one of the radiators, which we did, and the pressure went down to about 1. Problem is that although the pressure is at about 1/2 when the heating is off, as soon as we put the heating on the pressuree raises to over 4 (off the scale). Is there anything we can do to mend this, or do I need an engineer? Thanks!