baxi 105 e bolier loosing pressure no leaks ?

Hi my boiler looses pressure over night, the pressure gauge shows empty every morning if the heating is switched off over night.I was topping the boiler up every morning which resulted in over pressure.the hot water will not work as there is no pressure in tank, but if i turn on the heating, the boiler will repressurise its self, and after turning off the heating i can run hot water for most the day.The next day the same thing.
I have had a plumber look at the boiler but he is puzzeled as there are no leeks in the system ?