In particular 4.4 tdv8 but may be same/similar to 3.6.
Does anyone have diagram of heater matrix pipes to the rear heater core?
I have looked through the service manual but cannot find where they come from in the engine bay.

You’ve prob seen this already then. Can’t see much else....

Click image to enlarge

Yeh I saw that. Can't work out where those pipes attach near the engine bay.

So it would seem the coolant feed and return to the rear heater matrix are just taken from the front pipes.
Therefore my intermittent lack of heat to the rear must be at the rear heater unit itself.

Is the auxiliary electric pump running? If that's failed you would see poor circulation to the rear matrix.

Also, I'm pretty sure that the rear heater controls operate their own electronic coolant flow control valve, which if dicky would cause issues with rear heating.

IID tool would be able to tell you what is / isn't working correctly.

Jon 1992 RR Classic 3.9 efi Vogue
2014 Disco 4 HSE
===================
Both my fatties now gone...
Previous: 2011 L322 4.4 TDV8 Vogue SE /// 2002 L322 4.4 V8 HSE /// 2009 Discovery 3 2.7 TDV6 XS /// 2004 Defender 90 TD5 /// 1993 110 V8 Snatch Landrover /// 2005 Discovery 3 2.7 TDV6 SE (Aus) /// 1990 110 Isuzu 3.9 County (Aus) /// 1976 Series III Trayback (Aus)

Im sure the pump is running. After much investigation on the vehicle and parts diagrams, I’m sure the temp is controlled by the rear matrix itself.
It has a feed and return from the front, which gets hot as normal.

Next step is strip to get to rear unit.

Iid tool shows no live values for any of temp/actuators. I assume that is due to being 10-12 MY.

Ah, so if the supply pipe to the rear is getting hot, then that rules out the flow control valves in the engine bay.

I'm sure the IID gives Aux water pump status / current draw (I think I saw it on mine just the other day and I have a 2011). I'll have to have another look tomorrow for you. Maybe a firmware update is required on yours?

Your diagnosis to date does seem to point to the matrix itself, or some other local control valve on the rear matrix?

Good look with finding the fault, these things can be a pain at times.

Jon 1992 RR Classic 3.9 efi Vogue
2014 Disco 4 HSE
===================
Both my fatties now gone...
Previous: 2011 L322 4.4 TDV8 Vogue SE /// 2002 L322 4.4 V8 HSE /// 2009 Discovery 3 2.7 TDV6 XS /// 2004 Defender 90 TD5 /// 1993 110 V8 Snatch Landrover /// 2005 Discovery 3 2.7 TDV6 SE (Aus) /// 1990 110 Isuzu 3.9 County (Aus) /// 1976 Series III Trayback (Aus)

To follow on from this turns out despite ruling out the crossover valve as I had only a few months ago repaired it, turn, it was still faulty.

I repaired again and everything seems to be back to normal.

Glad you got it sorted, and apols for not getting back re. IID values - I forgot. But looks like all sorted now, which is cool  1992 RR Classic 3.9 efi Vogue
2014 Disco 4 HSE
===================
Both my fatties now gone...
Previous: 2011 L322 4.4 TDV8 Vogue SE /// 2002 L322 4.4 V8 HSE /// 2009 Discovery 3 2.7 TDV6 XS /// 2004 Defender 90 TD5 /// 1993 110 V8 Snatch Landrover /// 2005 Discovery 3 2.7 TDV6 SE (Aus) /// 1990 110 Isuzu 3.9 County (Aus) /// 1976 Series III Trayback (Aus)