sabre he boiler?

Hi hope someone can help with a problem i'm having with my sabre he combi boiler. Yesterdaymorning i noticed the pressue had dropped and red light was on so i adjusted the value thingy to bring pressure back up and when that was done the green light came back on. In the evening i put the heating on and it heated a bit and i went off to bed leaving it on low with thermostat on 17. This morning it was red again but pressure was ok so i turned boiler off and it flashed green again so i thought it was ok.
Then this evening the heating was put on and it heated up a bit then went red again so i turned it off and on again and was fine for about 1 minute then green light stopped flashing and just stayed green and pressure kept rising to well over 2 til i turned it off, please help just had a new baby and at the minute i can't afford an engineer to come and fix it so hope its something that can be done by myself - i'm quite handy at doing some things and will give anything a go. If anyone can help with advice or such would be very grateful, Vicki.