Reducing Economy 7 to 'Economy 5' etc?

Hi,
During the winter months when the weather is cold, my economy 7 works fine, however I have a problem that during the autumn and spring it makes the house too hot.
I've tried altering the input/output dials on each heater, but that makes little difference.
The only solution I can see to this, is to reduce the charge time, by installing a timer (by a qualified electrician) into the circuit. That would then enable me to set the charge time, based on the weather conditions/temp.
I wondered if this is possible, and if anyone has done this successfully ?
Thanks