vaillent Combi Boiler?

Hi,
I have the usual system needs repressurising every year, but now I have an 'over pressure', where my pressure is off the scale. I've checked the part where i normally repressure, to make sure that it is fully off, and am at a loss what to do next. THe manual only covers loss of pressure. Its been like this for a week, and doesn't seem to be doing any visual or audible harm. Any infor much appreciated

Anthony
July 2008