I've got the same problem with same boiler. My plumber says-- (1) Change the pressure relief valve, (the "3 bar" device), which is an easy cheap job. (2)--If this doesn't work there is a leak somewhere in the heating system. This can be a nightmare to find, but Fernox do a leak sealing additive so I'm going to try this next.
JohnC November 2007
John Cannell
November 2007