How can I fix my Worcester Combi 30si?

I have a Worcester 30si Greenstar Combi in my loft space .Last week we woke up to no hot water and no cent.heating.Checked the pressure gauge and found the pressure had dropped to '0' and adjusted this a little(i thought) to about 2,but in fact it was nearer 3.
The system is working ok(I am getting hot water/cent.heat.) but keeps cutting out and am having to climb into the loft again and again to switch it back on.My question is,"Can I lower the pressure back down to 2 and can I do this myself(is there a screw head I can adjust to do this) as I don't want to incur a £85+ vat.call out charge?