It sounds like the domestic heat exchanger has an internal leak, meaning that the mains pressure cold water, being a higher pressure than the boiler system pressure (1.5 bar), is flowing in to the system, raising the pressure. You can check this by turning off the cold water service valve underneath the boiler, and blasting off the pressure to 1.5.
The pressure should stay level over a period of time. Have the boiler switched off for this so that it doesn't rise with the heating coming on.
scott, December 2008
