How do you restore the pressure setting?

the pressure on the boiler stays between 2 and 3.5 and occaisionally rises above this. The problem started when the bathroom was refitted and the plumbers wrongly plumbed one of the hotwater pipes to flow through the boiler a second time causing the boiler to switch on and off frequently -this was corrected after they tried a few different solutions (I don't know what they did)- however the pressure problem still exists even thogh a new plumber rest the pressure by removing the air by draining one of the radiators- does anyone have any ideas as to what I should do? -Thanks