is my immersion heater broken

[mukaka]

I have an immersion heater to get hot water in my flat. it runs on electricity as i have no gas in the property. i have not been in the property long & my first electric bill was extortionate.. the company advised me to switch off the heater until about half an hour before i want a bath or whatever so i tried this today but after an hour of heating i still got a cold bath! does this mean my heater is broken? i can't get a hot bath uness the tank has been on all day and THEN i put the boost on for an hour!! help!