That sets how long the upper element will come on for if you push the control for a boost. At present, if you press that control (it's a rotary switch and push button combined) then the upper immersion element will come on for the shorter of :
- the time set - currently 2 hours
- the time it take for the upper immersion thermostat to operate (i.e., water is hot)
- you turn it off again
As for the timer, as Davesparks has hinted at, you really want it to switch on the lower element for the full time of your E7 cheap rate. The ideal would be if it came on just as your E7 period started, and off again just before it ends - that way, you can fully heat the hot water until the immersion stat turns it off.
In practice that's not trivial to do. You need to accurately sync the clocks (this timer and the E7 timer) otherwise you risk using peak rate lecky when you shouldn't be doing. So setting 1 hour in from each end gives you (as hinted at) a buffer. But you also need to check the E7 timer and make sure that it really is giving you cheap rate when you are expecting. Post a photo of that if unsure.
Better still would be to use a connection from the E7 clock to control your immersion - but that would need some wiring changes.
As an aside, contrary to what many will tell as as hard and fast
FACT, it does not cost you to heat the whole cylinder every day. Many will tell you that heating the whole cylinder but only using part of it each day will cost you lots - because they genuinely believe that all that hot water you didn't use magically disappears and you have to heat the whole cylinder again ? You will only have to heat the amount of water you use - so if you used (say) 1/4 of the capacity, then you'll only have to heat 1/4 of a tanks worth of cold water up to hot.
What is true is that
if you do not have good lagging on the cylinder, then standing losses will be higher if you keep a cylinder full of hot water than if you don't - but if you have good lagging then these losses are low, and if it's inside the house then it's not "lost" heat as it contributes to heating the house (except in warm weather then you aren't heating).
Also, in case you weren't aware.
The idea of the "boost" is to use an immersion heater that is part way up the cylinder. That way, if you are running low, you can heat just the smaller amount of water at the top - while leaving the lower part unheated. This gives you (a smaller amount of) hot water quickly, without having to heat the whole cylinder on peak rate lecky.
