A quick point about sitting in the car park on cold mornings: the engine will take ages to get up to temperature if you're not moving, and some diesels may never get up to the right temp. Wipe the windscreen and get moving as soon as possible, if it's sitting there idling it's not going to do the engine any favours.
Stupid questions out of the way first: have you checked the demister is working? Do you have air con, and does it need regassing?
You could try neat fairy liquid rubbed into the screen, probably a very similar thing to that rainex stuff anyway. Worked a treat on motorbike helmet visors, although you don't want it getting too wet!
Or keep a microfibre cloth or two in the car and give the windscreen a good wipe to help the demister.
You can buy some stuff from Halfords, sorry I can't remember the name, but same people that make rainex. It's liquid you wipe on inside of screen with a dry cloth, stops it misting up - works fairly well (def worth a try)
I have a VW Touran, brilliant BUT the inside of the front windscreen fogs up and takes for ever to clear, 5/7 mins in the mornings. Is there some way to stop it, control it or deal with it quickly that isn't the cuff of my coat? I am a first responder and need to get on the road quickly, not waste time waiting, sitting in the car park.