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Thanks for this info, v useful.
Have replaced the element a few years back - old one had no rusting - and the problem has continued at same frequency.
Thermostats replaced every 12-18 months, does seem to be during winter they fail with more frequent use. I’ll look at testing the failed one I’ve just taken out. The pattern is that once they trip, they’ll trip again in a week or so, then more and more frequently, until replaced.
CU has a certificate, and the person who installed cleared up a few other problems in the flat (1970’s build, legacy problems from previous) and this was flagged as an issue to test/address, but has continued since it was done, maybe 3 years ago.
Chopsicle-
It does sound like 'normal' failure patterns for a thermostat immersed in water but seems like the intervals are a bit short. On the other hand most electric immersions that I come across these days are only a backup for a gas heating option so they're not heat cycling that regularly.
I wouldn't expect rust after 12-18 months. Could be internal failure not related to water but some sort of water ingress is also likely given it's immersed in a tank.
I'm scratching my head to think of some way the wiring could be a cause but can't come up with one.
Airhead
Current draw should only be affecting the cables, the cables should be protected by the breaker. The earth resistance path through the pipe should be low as the pipe is a fair bit of metal.
If you've replaced the CU they should have tested the circuit resistance and importantly the insulation resistance of the wires i.e. is current passing between the conductors or from a conductor to earth because of faulty insulation as that would be a common cause for tripping.
The elements themselves are known to cause tripping because of insulation resistance failure but in this case it's mostly rusting of the element allows water in the tank to connect with a conductor? How often are you replacing thermostats and are they testing faulty when tested? It's pretty simple to test a thermostat.