how can i depressurize the boiler?

i have a vokera boiler compact24.two days ago o got up to find i had no heating or hot water so went upstairs to reset boiler . i now have hot water but no heating.i spoke to a plumber and when i explained that there was a shuddering noise he said it sounded as though ihad over pressurerised it.he then advised me to switch of the system and bleed the radiator leave for thirty minutes and turn back on . i have done this and still i have no heating can give me any advise to what this might be

cheryl
February 2012

using a bleed key, allow some water out of a radiator bleed valve and catch with old towel.
not quickest but easiest route generally for home owner.

the only thing to mention is why did it get that high??
owner over pressurise?
boiler creeping up?
rising when on ch?

iain
September 2008