"Rest" heater not working

[Avocet]

Hi All, I'm new on here.  We've recently bought a 2017 Kodiaq (7 seater 150 bhp diesel 4x4 DSG).  Due to lockdown, we haven't used it much, but I did have to undertake a reasonably long trip for a hospital appointment recently, and was disappointed with the "Rest" heater function.  I've had other VAG cars with the same engine and the Rest feature and it has worked brilliantly, keeping the cabin warm for up to half an hour.  On this one, when I pressed the button, the light illuminates and the heater fan runs, but what comes out of the vents is only lukewarm and even then, only for a few minutes.  I think the system works by having an electric water pump to circulate water from the engine around the heater matrix.  It's as if there's no hot water getting to the matrix.

Has anyone else had this problem?  I've checked all the fuses that I think it could be and they're all fine.  The car is covered under some sort of extended warranty from the dealer, but I'd be curious to know what the fault might be.

[Sargan]

I have  'Rest' button but never used it, didn't even know the purpose. 

I'll try it next time after a run.

[SuperbTWM]

Sounds like the pump to me, worth a diagnostic scan to check it.