Boiler not heating water

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I rent an apartment and for the last year the boiler has worked fine.
I used to have it set to heat the water every morning for an hour or sometimes 90 mins.
As I live alone I only really use hot water for showers.
But as electricity prices in the UK have doubled and I don't shower every day as I work from home, I decided to turn off the boiler and only turn it on when I need to heat the water.
As you see from the pictures, there are three mains switches below the thermostat controller unit. I just turned all three to off as I couldn't figure out how to tell the thermostat not to heat water. Then after I take a shower I just turn the mains switches back off again until the next time.
It's been going fine for a month or so. I just turn all the main switches back on then set the timer to run for 60 or 90 minutes and that heats plenty of water for a long shower.
But in the last few days it has failed to heat the water.
Today I set the timer from 1:30pm to 3pm but there was no water. It was very slightly, lower than luke warm at best instead of the usual very cold so it did heat it a bit. So I set it again to heat from 7:30pm to 9pm but again there was no hot water.
The thermostat makes me set three different times and I set all three for the same time to start and duration as usual. So that's 3 hours of heating in total but no hot water.
Does anyone have an idea of what the issue might be?
The level of the overflow tank is full and the boiler looks in very good condition.
 

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Assuming you are turning the switch on the clock to timed rather than off on the left hand rocker switch ,when it’s calling for heat so the boiler should run is the boiler running ? A photo of the boiler would be handy.
 
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Assuming you are turning the switch on the clock to timed rather than off on the left hand rocker switch ,when it’s calling for heat so the boiler should run is the boiler running ? A photo of the boiler would be handy.
I did notice that rocker switch was set to off but I don't remember setting it to 'off' myself. I just set the bottom three mains switches to off when I'm not heating water.
I just switched the rocker switch to Timed and set the timer again so see if it works.

Taking the cover off the boiler encasing is a bit of a pain so I won't do it just yet, see if this first change works.

Also, the boiler never makes any noise when it's heating water. Maybe that's because it's electric but I would have expected to hear the water percolating as it heats up. The only time I hear it is when it's topping up after I run the taps.

While I'm here and on that last point, I've been wondering something about how boilers work.
When the water in the boiler main tank is heated up and is full, then I run the hot taps or take a shower, use the washing machine etc, I can then hear the water being topped up. But surely that would mean that cold water is being poured into the reservoir of heated water and so diluting the heat/cooling it down even as you use it.
Or is there two reservoirs in there with the one that is always topped up being the cold and when it is heated it is put into a separate tank?
If it's the latter, then why does it make a water flow, topping up sound as soon as you use the hot water? Surely the water you are using is being taken from the hot water tank and you don't want water being topped up into that unless it's heated first. So the topping up water must be going into the cold water tank (if it is indeed a two tank system. Or is it that when the shower or washing machine or even hot tap is run, it always mixes the hot and cold so people don't scald themselves so it's always drawing water from the cold tank, hence the instant topping up?

Update: Yep, water is now heated. So indeed was the rocker switch on the timer unit.
 
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While I'm here and on that last point, I've been wondering something about how boilers work.
That particular Gledhill effort is a thermal store - the water inside is heated, but that water isn't what you get from the taps. The small tank on top is just to allow for expansion of the water and to replace the tiny amount that's lost by evaporation.

The water you get from the taps is heated either via it passing through a coil of pipe inside the hot water, or on the fail-o-matic older models via a plate heat exchanger on the outside.
 
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That particular Gledhill effort is a thermal store - the water inside is heated, but that water isn't what you get from the taps. The small tank on top is just to allow for expansion of the water and to replace the tiny amount that's lost by evaporation.

The water you get from the taps is heated either via it passing through a coil of pipe inside the hot water, or on the fail-o-matic older models via a plate heat exchanger on the outside.
So the initial heating is to convert the hot water reservoir into a kind of storage heater type deal, then the water that comes through the tap/shower passes through a coiled tube draws the heat from the reservoir?
Why don't they just draw the water directly from the reservoir as it's already hot?
 

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