******* FIXED - Well sort of ******************
I thought that I had an issue with the viscous fan ( it didn't seem to kick in & would spin easily). So I changed that.
I also removed the the themostat completely at the same time & would you believe it, I have heat all the time at 60 /70 MPH on the Motorway

However & here is the however, the engine is Not reaching operating temperate at this speed. I have had it down to as low as 56 degrees C (still with heat out of the vents).

Before removing the thermostat, I heated it up & confirmed that is statred to open around 85 degrees & was fully open at 98 degrees.

My question is what do I do now, Try another thermostat (that will be the third) or add some holes the existing one to allow a little more flow.

Thanks for the update on this one.

I have the same issue. perfect heating below 1,900rpm but as soon as the engine goes over 1,900 the air starts coming out cold, drop below 1,900 and it's warm again.

I'll follow your update here and take a look at the thermostat. Did you ever fully resolve it in the end? 2006 L322 Vogue SE 4.2

Hi, I bought the car off highroller last year and my mechanic found one of the coolant return pipes was fitted the wrong way round. I have hot air at all RPMs now! 😀

I’m not sure exactly which pipe otherwise I would let you know, but hopefully that helps. 2005 L322 4.4 AJV8

That is a help as I can go through and check the pipes against a coolant diagram.

Next is also fixing an intermittent heated seat, it's been working fine but started becoming intermittent a few weeks back. Must be a dodgy heating element. 2006 L322 Vogue SE 4.2