Just to follow up: now three months in, and everything is still working fine. It runs the immersion at the cheapest overnight time (I’m on the Octopus Agile tariff: got paid for the last two nights’ runs 
) for a maximum of about 35 minutes (cold tank to thermostat switching off), reporting a maximum internal temperature of 80°C. I’m careful not to switch the relay off whilst the immersion is running to avoid arcing — I have an automation to switch it off when the power drops below 100W for a minute.