Absolutely, but it's probably best not to turn off rads completely and remember that water vapour in the air will condense in the colder (unheated) areas of your house, so adequate ventilation is important. A more effective way to reduce your gas bill would be to turn down your room thermostat/ boiler stat by a couple of degrees, make full use of the timeswitch and avoid unnecessary heat losses through draughts, inadequate insulation etc.