I agree with Whales idea the problem is if it is off peak supply that heats the water it will take a couple of days to take effect (my tank takes 5 days before it’s noticeably cooler after switching off the heater).
However have a look at your tenancy agreement
9. The landlord agrees to pay:
9.3 all charges for the supply of electricity, gas (except bottled gas) or oil to the tenant at the residential premises that are not separately metered,
Your hot water is not separately metered so give the bill to the agent and tell him you expect the landlord to pay all power bills until the situation is rectified. You can also inform him (lie) that you will be away for 4 weeks in June and you will be turning off the power during this period (including power to heat the water) and he should inform the other residents that they will have no hot water for 4 weeks.