Well that's blown my theory out of the water.
I was thinking you were on economy seven electricity, and the night time heating element had failed, so you only got the daytime top up immersion heater working.
However that might not be too far from the truth.
Have you got an electric immersion heater element turned on? Perhaps the oil boiler heat is not getting through, due to a stuck motorised valve, so you only get a small amount of hot water at the top of the tank.
After someone has used hot water listen to the hot tank. If the electric immersion heater is on you might hear the sound like a kettle in the early stages of heating water.