Instead of hijacking multiple threads I thought I'd start one to capture it all

So I have a UK 2014 V8 diesel bought 3 years ago. The climate control in the cabin has always been a bit cold, but lately I've had enough! The rear passenger foot well has always been cold - JLS here suggested a possible flap issue? The rest was warm (I'm sure of it) but isn't now. I wondered if maybe the FFBH valve needed replacing, maybe recently failed, or the system needs bleeding.

I'll describe the situation and hopefully some guidance is forthcoming. It's 4 degrees outside.

I've had the car running on the FFBH for 30mins then I started the car and left it ticking over for 30mins (not ideal but I have no journeys to make!).

After this period if I set all dials in the car to 28 and "auto" the situation is as follows. My coolant light has came on and I topped it up prior. I do have a small coolant leak, and the rear coolant system has been looked at whilst resolving the leak but in the end it was a pipe in the engine bay.

Basically the only warm air (not 28 degrees but is warmer than any other) is the rear O/S floor vent. Almost everything else is cold or ambient (still cold due to outside temp).