I thought that I would look here for some inspiration to my issue as it's really giving me grief..
I have no heating either side, however some times it works fine, but mostly not existent. This has been an on going saga & basically the only thing left to change is the engine thermostat.

I have replaced the radiator, expansion tank, heater control valves, pump & Internal temperature sensors.
Bled & bled the system, Flushed heater matrix, All flaps operate & 3 red dots fully showing.
No overheating issues & no fault codes, control valves taken apart again & all ok with power to them, pump operates when powered up.

Top hose hot & bulkhead return hose to engine hot, Both control valves are hot, but pipes are cold, Then inlet pipes to the heater get warm, some heat, but then cools, Both control vales cool & all pipes cool.

Bottom Hose still cold after running & temperature gauge shows normal. I thought that the control valves operated independently of the main engine temperature, to ensure quick heat after a short period of running , however this does not seem to be happening.

It can only be two things now a faulty thermostat or although new a faulty control valve. (loathed to take that apart again).
Although 4.4 V8AJ petrol No FBH.

I think that I have pretty well covered everything but always happy to listen. Oh, just to add, I've had a JLR technician look at it as well & he's at a lost ( agreed with me possible Thermostat).

Taken it out for a run & all working fine, All heater pipes hot & heat in all zones

I've ordered a new thermostat anyhow. As I feel that the Thermostat is possibly staying open initially, thus causing semi cold coolant around the heater matrix instead of sending hot coolant when the Thermostat is shut.

I'll update once replaced,

I had this on My 4.4 V8. Even Craig at Camtech couldn't fix it. Anyhow you have covered most if not all of the possibles.

Heres what I did .....I went into the engine compartment and then removed the pollen filter and looked around for any impedence/blockages/causes. Then I replaced it and checked it was all seated correctly.
got back in started her up and blow me down wiv a fevver ..... it blasted out hot air!!

Think it may have been some dirt clogging up the gears inside the flap area which may have removed itself on my prying around.

Worth a try though!

************** Solved ********** :
At last I have solved my heating issue. Changed the thermostat & all sorted. The Seal had come away from the mounting plate letting coolant pass (i.e the thermostat was partially open all the time).

This was my last port of call & should have looked there first of all. This is why my heating has been temperamental. If the seal was stuck in the closed position then the heating would be ok & continue to be ok until the seal moved again, never really letting the engine get warm. Thus the temperature gauge never going above normal.

*** Future reference for any one with similar issues, Is to look at the expansion tank return hose (the one adjacent to the bleed screw). If this is flowing coolant when you initially start the engine, then the thermostat is stuck open.
As once I had changed the thermostat this didn't flow until the engine was up to operating temperature & the thermostat started to open.

Well done that man and thanks for posting the fix far to many posts on here with no resolution posted.