You need to check the flaps on the heater box are actually moving and doing what they are instructed to do, maybe a bowden cable has come off, assuming they have these, don't know them that well me self.
Phone 01953 458000 and ask richard,(tell him the fat guy with the cossie sent you!!) he does nothing but rovers breaking spares repairs servicing etc, so sure he's come across it before, lucky git has one of them ZT260 V8 ones!!
tabetha