I think it's a design problem, i have fitted a new valve the heater hose from the valve was next to cold and I still had heat from the vents, i then fitted an internal non-return valve to the heater matrix return line, before it joins the valve bypass hose , this makes some diffrence but i still get slightly warm air which is a good improvement to as it was.
I have just had the aircon regassed and now i get cold air. but i have it in mind to change the valve position and type of valve, but i don't think it will get much better.
The air temp after regassing struggles to get lower than 3.9C but was better than 8.0C before, so the system is too small and the reasons that VMax 1000 gave, but every little improvement helps.