Post the actual figures from the bill, units and rates, and the name of the tariff they are on.
If you have only just installed the storage heaters, they won't be old school real storage heaters, the EU outlawed those and even though we have left we have not reversed that. So they will be some fancy modern combination of part storage and part convector and will use a certain amount of peak rate daytime electricity to top up the heating when it's control algorithm thinks is best. Perhaps you can post pictures or a model number of the heaters?
And are they even that sophisticated, or were you hoodwinked into getting a certain notorious brand of electric radiators with a clay core that have sometimes been described as "storage"?
Locked to prevent being hi-jacked (again)