Thats not really taking into account the thermostat requirement, and as far as I know you dim the mains supply going into a thermostat the thermostat will fail to work. Using two dimmers in a circuit will cause you more problems than its worth. Habistat do a dimmer thermostat with a timer or a magic eye, http://www.cornishcrispa.co.uk/day-night-dimming-thermostat-p-960.html gives you an idea of cost and what it does/how it works.
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