Worcester 24i presure probems?

My worcester 24i combi boiler keeps losing pressure over a fairly short period of time, and I have noticed when I'm filling the bath with hot water the bathroom radiator is getting warm which is the first rad in the system and the closest rad to the boiler, which means the hot water side of the boiler is getting into the heating side, which logic tells me the loss off pressure on the heating side is the water going back the other way into the hot water side, I just can't seem to work out where in the boiler this is happening, can anybody help with any advice please?