Ford Puma W Reg - Heater system blowing always blowing hot air?

Hi, hope someone can help us...

We have a W reg Ford Puma on a 2000 plate.

The heater system is blowing hot air constantly.

My husband is a mechanic but this is boggling him.

We purchased a Heater control valve as we read that this is what it could have been.

This arrived this morning, he has fitted it but it hasn't cured the problem?!?

Does anyone have any idea what this could be? or how to fix the problem?

Many Thanks in advance