For the average sized installation most combi boilers should be pressurised to 1 - 1.5 bar when cold. You would expect to see a rise of up to 1 bar under normal conditions. Rises in pressure above this range (usually apparent within 20 minutes) indicate a fault with the Expansion vessel which may need re-pressurising or replacing.
If you don't understand what I've written, then it's best to get someone in. Not wishing to sound patronising, but you don't give a clue as to your level of competence.
Gas Safe Engineer in West Yorkshire, November 2013
