Hi all, just wanted to let you know that the problem has been resolved. I contacted Southern Electric and they sent an elecrician / engineer out the same day. He tested the electrics and found that one of the wires (connected to the offpeak devices) was not receiving any electric. His solution...
I've also taken a note of the meter readings this evening (as suggested by someone in another topic). If the off-peak figure hasn't changed by tomorrow, would that confirm that it is more likely a problem with the meter?
I leave them on 95% of the time so I only need to use the switches downstairs to turn the hot water on and off. Left leads to the top of the water tank (booster), rights leads to the bottom of the water tank (overnight)