My kingdom for a pressure gauge on the HW cylinder, it would save a lot of hassle and expense for people if it was included as standard on all unvented HW Cylinders.
Your findings are very interesting in that your plumber saw the pressure rising/falling by 1.5 to 2.0 bar to 4.5bar, if, which seems the more likely explanation at the moment, it is flowinng back through the shower & HW cylinder expansion valve then down the tundish then it points to either the E.valve (or the cylinder safety valve) are gone weak and lifting at 4.5bar or the E.valve has been set too low, also if the HW cylinder PRV has been set to 2.8 to 3.0bar then the E.valve could lift each time a big volume of water is reheated, admittedly to only discharge ~ 4 or 5 litres each time. Also loosing say 400L at night through the HW cylinder should mean that the whole cylinder had to be reheated each morning from practically stone cold, did you notice anything strange about the heat up times?
Your findings are very interesting in that your plumber saw the pressure rising/falling by 1.5 to 2.0 bar to 4.5bar, if, which seems the more likely explanation at the moment, it is flowinng back through the shower & HW cylinder expansion valve then down the tundish then it points to either the E.valve (or the cylinder safety valve) are gone weak and lifting at 4.5bar or the E.valve has been set too low, also if the HW cylinder PRV has been set to 2.8 to 3.0bar then the E.valve could lift each time a big volume of water is reheated, admittedly to only discharge ~ 4 or 5 litres each time. Also loosing say 400L at night through the HW cylinder should mean that the whole cylinder had to be reheated each morning from practically stone cold, did you notice anything strange about the heat up times?