potterton performa 28(condensing) combi?

why does my combi keep losing pressure?? i have no apparent leaks anywhere and i know there is about 0.75 bar in the pressure vessel at the rear. There is no water being lost out the pressure relief either???
Also another problem is that my heating comes on by itself? This has just stared happening and i don't think it is related. The boiler is about a year old.