This is most likely a faulty element, but it may also be a tempature thermostat fault.
Your hot water system has two heating elements, one located at the bottom for normal heating, and this has its own thermostat, the second element is located towards the middle of the tank as is to provide a boost of hot water and will also have its own thermostat.
It is for this reason why overnight the water isnt heating up as the bottom element isn't coming on, but when you turn on the boost the water is heating up.
Depending on the exact model you, have the thermostat and element may be one piece. A replacement element and thermostat would be around £120-150.
I would suggest you have this replaced by a suitable trained plumber, if you would like me to recommend someone please provide your postcode.
PS: I assume you mean Saida system not Sadie?