1) an immersion heater should not go into a plug and socket, because it is a fixed appliance that will draw its maximum load for frequent and prolonged periods, so should have a dedicated circuit.
Hello again, John! The reality though is that many users do have immersion heaters attached to a plug (I myself am guilty) and the timer (plugged in) that I use actually stated on the package that it was suitable for this use. In my case it's on for a couple of hours during the day to top up when the boiler is off. It's on an upstairs main with just clocks etc. plugged in and I don't suppose there's much danger of overheating.
There has a debate going on for decades as to whether it is better (more economical) to turn immersion heaters on and off or to leave them on 24/7 (with their thermostat doing the controlling). If the cylinder is well insulated, then I suspect that there is not much difference in running costs (obviously only switched on over weekend if, as you seem to imply, the flat is only occupied at weekends).
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