I had economy 7 in a flat I rented and the time was out by about 8 hours. This was more convenient as all electricity was cheap from about 5 pm to midnight which was far more useful for cooking, laundry, showers etc as I could do it all on the cheap after coming home from work!
If you have the more advanced thermostatically controlled storage heaters, they will automatically adjust for warm and cold spells of weather by heating the bricks less and more to achieve a pre-set ambient room temperature on the charging cycle. Basically pick your input setting on the dial and it (the heater) will do the rest, If I remember correctly number 4 related to about 18/19 degrees C. The output dial is best left at it's minimum setting until the room temperature drops, usually around 12 hours after its finished charging (the end of E7 time), then increase the output setting to a higher number. This opens a vent to allow heat out of the storage heater, the higher the number the quicker it releases, remember to close this vent (turn back to minimum) before retiring to bed as the heat will escape prematurely the following day after the next heating cycle.
Hope this helps.