The 'plumber' needs to replace the pressure release valve, or PRV. Every combi boiler has one. For some reason, once these things have opened, they rarely seal themselves again, which is a complete bore. The valves cost around a tenner - fitting time? Unknown.
Plumber man should also check that your expansion vessel has around 10 psi of air in it, and the system needs to be at low pressure when it is being filled. Thats it, in a nutshell. I'm no fan of the Glow worm, and if it does need replacing, consider the Vaillant Ecotec range. Good kit.
John
October 2009