Email through this morning that flexible IOG night time rates are dropping to 5.2p/kWh from 1st April which swings the maths even further in favour of storage heater.
I agree with @adriandorr though that a heatpump provides a better and more controlled heating than a storage heater.
We have battery storage which generally covers the heatpump usage. 95% of our grid usage is therefore in the off-peak period so electricity cost will be dropping by about 25% with the new rates.