Ridiculously High electricity bills, am freaking out! Help needed

Shedman

OP what are you using the water you are heating using the immersion heater for?  If its just for washing (self not clothes or bathing/showering) and/or washing up it would be a lot cheaper to boil a kettle.  Do you have an electric shower?  Could be cheaper than using immersion but keep showers short .

My stepdaughter's in an all electric 2 bed flat with panel radiators (1970's build, south coast location).  She doesn't use immersion but has electric shower.  Fridge, washing machine, occasional tumble dryer use, electric cooker (but don't reckon that get used much 😅).  At this time of year (Nov - March) she is using around 20-25kWh a day (at capped rate thats about £5 a day (£150 a month).  April to October its only about £25-35 a month.

Even at 57kWh a day (which is definitely on high side) you should only be paying around £11 a day or £330 a month so not sure why you think it'll be £750 a month

Armengar

switch everything off for a day (other than fridge/freezer) and see.  You might have a faulty panel, upgraded immersion heater, who knows. If you just moved in then you have no comparison so anything could be dodgy, including the wiring.