I had to call a plumber out yesterday and while he was here he checked my immersion tank.
I live in a block of 4 rented flats. It seems we all share the same tank. I laughed dryly and said "how does that work with hot water bills then?" And he said there's no way to track who is using what so the bill is divided between you. If someone happens to be a high user then the others effectively subsidise their bills.
Especially now with cost of living rising, I'm surprised that this can be a "thing" and that they can get away with not telling you when viewing and applying. I've lived here three years but have had periods when I haven't lived here (six months over lockdown when I only popped back to get post etc for example), and I was surprised when my electric bill was the same as usual after that time. Now i know why! 
There's nothing I can do, but this week has been really shit already and we're only on Tuesday so this revelation has pissed me off even more than it would normally.