how do i kep water pressure constant on a worcester bosch 230 boiler system?

i get the boiler pressure up to 1.2bar by opening up the small valve next o the flixi hose pipe. Once pressure is at 1.2, i close valve. system operates as normal, but after a few days the preesure has dropped off quite a bit, sometimes to 1bar or just below. What could be causing this???