How can I fix my ferroli combi boiler?

My 2 year old Ferroli F24 combi boiler keeps losing pressure. I refill it to 1-2 bar, however the pressure keeps rising to 3-4 causing water to leak down a pipe! where is this water coming from? Is it a faulty pipe? I'm reluctant to call engineer out in case they quote £100s of pounds and nothing changes. Adding to this problem is the fact one of the loop taps has snapped (plastic fitting!!). Is this fixable by me? or should I get a professional? Any help or tips ?