As well as the useful repy from Brian there is also an Eon reply in this thread which gives indicative unit prices...
This is not an argument againts Heatwise, its a "comfort" tariff but the Heatwise unit price has a premium over what you would pay using Economy 7 only. Its impossible to say what's most cost effective in every case. Storage heaters are designed for 7 (or 8) hours charge not 10 hours. If you have a very well insulated house Economy 7 without Heatwise will probably be cheaper. Heatwise will be warmer but more expensive. A difficult calculation which depends a lot on your usage and insulation standards.
You have several choices, Heatwise + Economy 7, Heatwise + single rate tariff or Economy 7 only. Unless you use a lot of appliances overnight I am not convinced Heatwise + Economy 7 is the most economic choice. Compare the indicative unit prices on the above link and also compare your annual heating and non heating usage using a comparison website. Economy 7 can be compared using a comparison website, but not Heatwise unfortunately.