Landlord sent some electricians to my place end of September to check some stuff. Dunno what, but they had to have access to every electrical point and switch. They came early in the morning, I left for work while they were still there after showing them where everything was. The last thing I showed them was the immersion switch in the airing cupboard. I specifically told them I never use it, and it was turned off when I left. It's a fairly large electric water heater used for bath water and the hot tap in the bathroom sink - I have a separate electric shower so don't need it. I use the cold tap for my teeth and my face, and the shower for everything else.
I never look in the airing cupboard either, it is tucked away in an awkward corner and kept shut since there's nothing else in there but the heater.
I bumped against the door last night and it popped open. The immersion light was on! ie, the electricians had switched it on to do something, but they didn't bother to switch it off again, and it has been on 24/7 since then.
I just looked up the cost of heating a 120l tank (don't know the size of mine but that was the smallest option I could see on any of the online calculators). Based on 2 heating cycles a day it costs £850 a year, or £75 a month, to run an immersion tank. 
So it's cost me probably at least £100 because they couldn't be bothered to switch it off. Since my tank is not on a timer or anything, it could potentially be far more.
I'm fucking fuming but dunno what to do other than, I guess, complain to the landlord. Yes I know I'm an idiot for not checking, I genuinely forget it's there most of the time. I didn't even realise they'd need to switch it on since no one told me what they were doing!!
Suck it up or say something?