Flybe DHC8-Q400 multiple flight instrument display failures in heavy icing conditions at night subsequently attributed to the failure of the flight crew to select either normal or standby pitot and static heaters on after their failure to correctly action successive relevant checklists followed by descent with ATC assistance to clear icing conditions after which all displays returned to normal and the subject heaters were noted to have been off and were correctly selected; Glasgow UK 10 December 2006