Combi Boiler Pressure at zero when not operating?

I recently had a repair on my combi. The hot water kept going cold - not very pleasant when taking a shower! The corgi man came and changed something which cost me £257 and then a couple of days later it still did the same thing. He came back and fixed it again with another part a thermistor, I think. That cost me another £50. Do they know what they are doing? Now the boiler pressure goes down to zero when it is off but when it is operating it goes to just above 1. I fill it using the tap. I once discovered a lot of water on the floor as though it had over pressurised. Is it normal to go down to zero when off or have a got a leak somewhere I don't know about? I've looked at all the rads and they are not leaking.