That drop in temperature shown is normal I’d say as the temperature sensor is probably fairly low down on your hot water cylinder. I would think you should only need to heat once using the cheap rate electricity. That’s what I’m doing now. I used to have a second heat up at 3pm, when the air is warmer so a bit more efficient. 
It does trigger a reheat for me sometimes after a third shower. My reheat triggers when the temperature at the sensor drops below 30 and it heats to 40. The temperature of the actual water from the tap is still hot, I haven’t measured it though. 
I should think your dishwasher and washing machine probably only have a cold fill so won’t be depleting your hot water supplies. 

Just to add to Julia’s helpful remarks, I’d say that it will depend also on how big the hot water tank is. Given that the average British shower uses 50 l of water, at a guess you’d be using 35 l (70% of 50 l)) of the hot water. If the tank is a reasonable 100 l for a small household, a shower will use the top third of the tank, the bottom third will be refilled with icy cold water and your graph shows what a sensor lower down on the tank would detect. So if this is anything like your situation, it looks like the system is working as designed. The only thing I could suggest is to change the reheat trigger, to act at say 20 or 25ºC. As Julia says, there should still be hot water for washing up in the top of the tank, and possibly enough for a second shower.

Trial and error is the name of the game 

Good point Firedog. With my heat pump installation I have a 210 litre tank, larger than the old one as the hot water is no more than 50 degrees (excluding legionella cycle once a week in theory). 

Hi @Fawkon,

It looks like our volunteers have this one covered with some excellent suggestions and insights 

Keep us updated with your findings! 

Thanks all, will make the shift to the cheaper electricity window today and report back on how it works out in the coming weeks.

Hi @Fawkon 

I think @juliamc suggested that the temperature sensor maybe nearer the bottom of your tank. and that, cold water enters into the bottom of the tank “pushing” the hot water out of the top (hot water is less dense so it floats above the colder water).

Each time you use hot water the boundary between cold and hot moves up the tank.

A while ago I fitted four temperature sensors to our 160l hot tank so I could get a better idea what was happening.

At 18:05 you can see the first shower. The bottom of the tank cools quite quickly as cold water enters, however there is no noticeable change at the top of the tank (where the outlet is).

At 18:35 you can see the effects of a second shower. The lower sensor (corresponding to a usage of 60l) now start to fall, however the temperature at the top is still unaffected. There is a slight effect to the upper (100l used) temperature. I would guess that each shower uses around 50l of hot water. So we might get away with a third shower before the water actually turns cold.

If you know the size of the tank, and the position of the sensor you can work out how much water you need to use before the system will attempt to reheat. Measure the distance of the sensor up from the bottom of the tank and the total tank height.

Volume before reheat = Tank Volume * height of sensor / height of tank.

With a bit of luck this will work out as around 80l (more than one shower 50l, and less than two 100l).

This is the volume you use before the system will try to reheat, not the useful volume of hot water.

Finally because you mix cold with hot to get a nice shower temperature (43°C ?), the hotter the tank temperature the less hot water you will use. But remember hotter water means lower efficiency from the heat pump.

So even if the system says the water is cold you may still have lots of hot water left!

Only trial and error (or loads of sensors on your tank) will get you to the optimum.

I also suggest letting your partner have the first shower because if you get it wrong…...

Thank you @BeePee, fantastic information and insights, thank you!

We have trialled, for 2 days now, heating the water at 3 am with our 7p rate and we both had our normal length showers in the morning whille I also contiinued to test the water temperature at 10 pm each night and voila! The water is still warm enough for a hot shower late into the night if needed.

Hot water is now costing me a whopping 14p a day!