cabin temperature in my 456

Now that the weather is getting hot in the UK, the following problem became apparent. When I switch on the air condition hot air is flowing in the cabin. The aircon works just fine as a unit though.
The problems seems to be a water valve which is attached to the firewall (right next to the battery) and regulates hot water into the cabin ventilation system. The valve seems to be sticky allowing too much hot water at all time.

Anyone had similar issue?
Does anyone know the part number?

 

Yes, basically I had a friend who knows a things or two about it and he manually blocked the pipe that feeds the hot water through this water valve and amazingly a couple of minutes later the cabin was blowing with super-cold air.