Ravensheat boiler?

I have a ravensheat combi boiler.

every few days the pilot light will go off and the boiler therefore will not start.
when looking at the pressure switch it has gone to zero. i open the tap and add mains water into the system that then takes the pressure up. i have no leaks in the system and the water then overflow over a period, through the overflow system.
before this happens i start the pilot light and everything is ok again until the pressure drops and it goes off. i then repeat the same steps.
appreciate some ideas on this i believe it could be the pressure switch and if so is it easy to change?