water pressure?

I have had a plumber(SAFE REG) to change 2 pressure values thinking the 1st was faulty but it is still the same .Fill my system to 1.3 then after system has heated up and timed out I find that the pressure reading has dropped to 0.6 this has happened on both valves and the original one.Heating will not start up. Water is running out of my overflow pipe outside of house .There is no water from any of the rads ,have checked pipes and carpet.My Model is Themaclassic F35E Fanned flue combi boiler Is there anything you can suggest b4 i call plumber back out Thank you. Oh!Excellent Boiler otherwise 1st class