it is possible that when the heating is switched off over night the whole system cools down and therefore joints contract causing small leaks. these leaks will not show up when the heating has been turned on because every thing expands when hot. Try turning the boiler off before you go to bed and turning the flow and return valves off and in the morning see what the pressure gauge is reading, turn the flow back on first and then the return and you may get an answer where the leak is.
AB, May 2008
