How can I fix my Vaillant turbomax 828 ??

A certificated plumber recently made a repair to my Vaillant boiler, after which the unit appeared to be working ok apart from the pressure gauge which was soon indicating unacceptably high pressure in the system. He explained to me that the expansion chamber needed a top-up with air which he proceeded to do, that completed, he picked up his tools and said goodbye, unfortunately the pressure gauge hasn't worked at all since he left!. My calls to his number are being ignored, is it safe to continue to use the boiler in this condition?