Immesrion heater / thermostat problem

Premier_2

Yep, it sounds like you've almost got it.

The thermstat temperatures you suggest are about right.

If you don't have E7 (or other multi-rate tariff), then it doesn't matter when you heat up the tank.
The boost will only supply a small amount of hot water, enough to was dishes, but not enough for a bath usually.

As I mentioned, if no hot water is being used i.e. an open hot water tap (or shower, hot fill washing machine, etc) then the hot water tank shouldn't be filling. It only fills as water is removed.

If you hear water filling a tank after the hot water tap is turned off, this could be water filling a cold water header tank. These are usually in the loft, but are sometimes located directly above a hot water tank (especially in flats)