Vokera Compact problems?

I have a vokera compact boiler. I recently noticed some of my radiators needed bleeding as they were not getting hot. I bled them and re filled the pressure on my boiler to approx 1bar. Since then when the heating comes on with the timer everything works as normal except after it goes off the boiler loses pressure and I have no hot water. This happens every time the heating comes on and I have to refill to get it to work again. I have noticed the overflow dripping when the heating is on. Is there something I can do to fix this myself? I am fairly practical and like to have a go at what I can myself.