hi, for low pressure problems you simply need to top your boiler up with water from the cold water inlet pipe.
On the cold water inlet to the boiler theres a small, usually black tap valve...it should remain off (90 degrees to pipe)....however if you have low pressure simply turn the tap on and it will bring your boiler pressure back up! the required pressure in a boiler is between 1 and 1.5bar. once you reach this correct level, turn the tap off again until the pressure drops below 1 again.
reason for a loss of pressure can be a water leak either somewhere on your internal pipework or in your boiler itself...would be good idea to get it looked at by qualified plumber!
hope this helps
Stu
April 2008