dropping pressure on Vallaint 242E?

I have a Valliant TURBOmax VUW 242 E on which the water pressure is continually dropping. At first I thought that this could be a leak in the overall heating system and added leak gel to the system. I am now beginning to doubt this as after the number of times I have recharged the system if there was a leak it would surely be showing somewhere as a puddle! My thoughts now are turning to the diverter valve as now after several days of this I have noticed that when there is no use of the hot water (overnight) there is very little pressure drop. As soon as there is heavy use of the hot water (showers first thing in the morning) the pressure drop is considerable. I wonder if the diverter valve doesn't close off the heating circuit properly as it changes to hot water allowing the pressure to bleed out? Does this make any sense as the diverter valve is a known weak link - this is my 3rd and is now 2 and half years old. . . .