2003 Vauxhall Vectra blowing cold air

Heater problem
2003 Vauxhall Vectra Two Wheel Drive Manual 103000 miles

hi, when stood still my temp gage goes up and I get warm airbut as soon as I start driving my temp gage goes down to zero and I have no heat.
i have flushed the rad system and then topped it up again. All the pipes are hot so water is getting through ok but problem is still there !

can you help please

thank you