Is it just one immersion heater, or is there possibly two (lower and higher, possibly marked Sink and Bath)? If only for backup and there'd still be one left, I'd go with disconnecting. Otherwise I would try to change the thermostat myself - easy enough - and I'd probably try these guys, who are quite helpful...
https://www.heatrod.com/industrial/industrial-controls/thermostats--controls.html - or just hire someone. The fact that there's a working boiler right now doesn't help in the dead of winter when it breaks, of course... then you need to heat that water another way - the whole idea of a backup, really. That it may not have ever been used, that it may not even be understood... well, that's your critical decision to make, but probably in tandem with the Tenant, I'd suggest.
Landlord Forum
