First, what changes with the season alone
In August the mains water arrives above twenty degrees. In January it comes in near twelve. To reach the same shower temperature, the cylinder has to raise it by almost twice as many degrees with the same element.
The direct consequence: the same tank yields fewer minutes of shower, and takes longer to recover between two. Before assuming a fault, it is worth knowing that part of the problem is simply the calendar.
Limescale on the element
This is the number one cause around here. The water is hard, and scale settles on the element forming an insulating crust. The appliance uses as much or more, heats more slowly, and eventually burns the element out.
The symptom that gives it away is taking longer to heat than it did two years ago. If it also makes a kettle noise on start-up, the crust is already thick.
The thermostat and the anode
A drifted thermostat may be cutting out at fifty degrees when you believe it is at sixty. It gets checked and adjusted at a service, and sometimes that is the whole problem.
The sacrificial anode is the part that corrodes instead of the tank. Once it is fully consumed, the tank is what corrodes, and at that point there is no repair. Replacing it when due is what separates an eight-year cylinder from a fifteen-year one.
When the size has been outgrown
A cylinder sized for two people does not behave the same when the house holds four, or when a rental goes from two showers a day to five. In winter that mismatch is amplified.
It is a calculation, not an impression: number of people, habits and inlet temperature. Do it before buying a bigger unit, because sometimes adjusting the thermostat and descaling is enough.
What you can do
Check the set temperature, listen to the start-up, and note whether it takes longer than it used to. With those three facts a visit goes straight to the point instead of starting with diagnosis.
What not to do is turn the thermostat to maximum to compensate. It speeds up scaling, raises consumption and creates a scalding risk at the tap.