Hi Gaz.
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.
Phil.
