I have a 2012 TD4 and when I load up the electric demand (screen heaters, seats, lights etc), there is a sound like a slipping fanbelt whine coming into the car. When I toggle the front electric screen heater on / off, the noise comes and goes.

Where is my problem and what is the fix?

Anyone?

Hi
I had a sqeeling noise, that turned out to be a air con clutch bearing
When ac turned on or off you could easy tell the difference
I replaced the bearing in the clutch

If it's electrical load dependent, then it's either the accessories belt or its tensionner.

Ok, pretty sure the noise I am hearing is the accessory belt slipping when the engine is cold and there is heavy current demand / loading.

Is it a simple fix to tension it or is it a dealer job?

Buy a new accessories belt. It's cheap.
Easy to install. Just rotate the accessory drive belt tensioner clockwise to release the tension.
You can access it by removing RH front wheel inner plastic cover.

Awesome, I'll do that thanks.

Is the tensioner just spring pressure resistance? Or is it tensioned like the old fashion fanbelt by leverage whilst tightening of the alternator bolt etc?