high pressure on Worcester-Bosch 24CDi boiler?

Pressure on gauge rises very quickly, has even reached past 4 on several occasions. have to bleed radiator/s twice a day to control pressure. at 1.5/2.0 . Engineer we called didn't seem too worried and did not tell us what could be done or what fault may be. said water would run out of overflow. Anyone know what the problem could be? We run on LPG, does that make a difference?