Not enough Hot water (Pulsacoil A)

Hi, hope someone can help shed some light on this problem.
The boiler worked fine up until a couple of weeks ago. The previous tenants had sufficient hot water for showers, baths and washing up without having to use the boost button. When they moved out they turned off the power to the boiler (not sure if this is related to the problem). Since turning the boiler back on there is only enough hot water in the morning for a quick shower before it goes freezing. Pressing the boost button heats up more hot water but this only lasts a short time before running out.
I have checked that both the On and Off peak power switches are turned on. The top-up cistern is 3/4 full and the indicator light on the front panel is green (so i assume it is not a burnt out relay). The LED display on the circuit board is showing everything is normal. Could it be a faulty Off-peak immersion heater - if so, wouldn't this show up on the LED display or status light? Can anyone suggest any other things to check?
Thanks, Paul

off peak supply is usually controlled by a 15 amp fuse or breaker in the fuse board, if you have storage heaters and they are heating up ok, its usually in the same fuse board, i would check to see that its not blown.
hope this helps
mark.
www.gledhill-repair.co.uk