I have not seen a Horstmann boiler, in the main they made timers and thermostats, from the early to the modern types, but they simply controlled which item run when, what has changed over the years is methods to use Economy 7, early days we had the 'White' meter which only had power for 7 hours at night, today we have the smart meter, the AMR (automatic meter reading) has been with us for years, the new AMI (advanced metering infrastructure) is the next stage (the smart meter) the main difference is these enable two-way communication between the meter and the central system. So they can be used as a pre-payment meter (It has a way to turn off power) and can have power outage notification.
However they have moved the responsibility for switching items on/off from the supply company to the user. So clocks need to be set correctly, if your clock turns on power at mid day instead of mid night it will cost you a lot. The typical arrangement uses two immersion heaters, shown here with a timer controlling both, sorry link removed it would not let me post. there are a few ways of using it, one is to have top immersion heater set lower to the bottom one, so it will only activate when you have run out of water heated over night.
My controller can also detect when I have spare solar power, and can also use two immersion heaters, so there is no one description fits all. In the main unless some one has a bath a 40 gallon tank of water can last two days, however if the top immersion is being held off by the over night heat from the bottom one, then to reduce the time the bottom one is on could cost you a lot of money.
My daughter made this mistake and reduced the tank temperature with the gas central heating, so electric kicked in, and bills went up not down.
We aim to heat to over 60 degs C once a week to prevent legionnaires, but 40 degs is better at the taps, with a 7 day control this may be possible, but my iboost+ is far more expensive to a simple timer, so it is unlikely you have the option. As even the iboost+ still uses the immersion heaters built in thermostat.
Warning the immersion heater thermostat access normally exposes live contacts and is not really designed for DIY tampering.
I am an electrician not a plumber, so there may be things I have missed, however I hope you can see with my reply how important pictures are, so post a picture and you can expect more personal replies. Oh it seems it will not let me reply with pictures, I will remove pictures and try again.
Pictures replaced seems it was the link to description of how timer works.
