Water Heater Problem

I have a horstmann electric water heating controler it has a on button a cancel button and 1 led when the on button was pressed that was all you had to do and the water was kept hot all the time now whenever a bath is ran or the washing machine has been on I have to go and turn the on button again and wait for ages for the water to heat up.Is this a controler fault or could it be the thermostat in the boiler that is faulty
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