Our boiler keeps loosing pressure so that it goes below 0.5. I top up the pressure to 1.5 but it goes back down over the period of 3-4 days now. Can someone suggest checks to make before I have to call out a plumber? I have bled the raditors and checked for leaks to them and they are fine. Why is the pressure dropping? Could it be we need a new expansion vessel? The boiler is 10 years old and passed it's service last August.