So you rip out the off peak supply and the storage heaters that are designed to use cheap rate overnight electricity, store it in the heaters and release the heat during the day.
You install in their place "24 hour" heaters, which are presumably panel heaters that they can turn on or off at any time of day, using electrcity at the PEAK rate.
And both you and the customer us surprised the cost of heating the house has gone up? :C :shakehead