please help! worcester combi boiler pressure drops

hi please someone shed some light on this for me
i have a worcester combi boiller that is fine when running on the timmer but when it turns off the pressure drops below the line and the whole system turns itself off and red warning light comes on. to restart it you need to raise the pressure above the line and it ctarts again but the clock always goes back to 12.00 this started when i bled a radiator in the house that was cold at top corner dont know if thats conected but thought i should say
please excuse my explanation i know nothing about boilers!!

Sounds like a problem with the expansion vessel. This is dealt with in the FAQ topic near the top of the topic index list.

im sorry but ive looked all over the site and cant find where you mean in the faq or anywhere can you explain for me please!!!???????
thankyou so much