Valiant ecotec30 keeps creeping into the red?

Hi, my new boiler seemed very low on the pressure and so i released a bit of water into the system to bring it up to 1.5. This seemed ok but later that day it went bang into 3.4 on red. we immediately drained water from the system and brought the pressure down but it has twice crept up again - any advice, and what should the pressure ideally be at for when it is on/off? Thanks