Have a feel of the two heater hoses at the engine bulkhead,if they are both warm/hot then you may have a heater control/flap fault,not sure but on some Vauxhalls there is no control valve and it is regulated by diverting air from the outside via flaps. Have had problems on a Vectra also where the heater radiator was blocked itself and needed flushed out. Good luck.
cdc, April 2016
