Hi Gaz.Another issue. Cold air is being blown out only. The car was fully up to temperature. What could be the cause?
There are many posts documenting the typical heating issue found by so many.
Of course, they may not apply to the Diesel model, I really don't know.
On my V6 petrol, I had to disconnect the heater hoses running along the inner wing O/S/F.
They will lose coolant of course, but that's inevitable.
Run a hose pipe to the car and connect to one of the heater pipes - crap should flow out of the second pipe.
I repeated this process using the opposite pipe to back-flush the heater matrix.
It was suggested that a cleaning chemical could be used for stubborn blockages.
I read somewhere that Steredent tablets in a solution also helps clear calcium build-up.
If this method fails, the matrix may have to be replaced - it seems quite common on these cars.
I believe someone has done that change from inside the car via the glovebox aperture.
If I read it correctly, the alternative is a dash removal (painful).
Try the matrix flush - it worked for me. Don't put radweld or similar anywhere near the coolant system.
Good Luck buddy.