In reply to kevin stephens:
Stayed at the cairngorm top car park a couple a weeks ago with no issues. The police drive around at about 2000, I suppose to check for stragglers. There are normally about 10-15 vans parked there overnight on a weekend. It was -6 up there in the van as I didn't have my heater on overnight, so getting up was painful. If you look into the mountains you can often see the flickering of headlights until well gone midnight.
Just beware if there is going to be a big snow dump, they might struggle to open the road in the morning (if say you need to get down mid morning for what ever reason).
You can just about get phone signal with three mobile aswell, which you can't get in the lower car parks that you also have to pay for.
The flatest bit of the car park (hard to tell when it is dark) is on the left hand side by the lift station (basically where most of the vans are parked)
No showers available up there for public use, so if staying for a few days. Best bet is to go into aviemore and pay £2.30 for a shower at the new sports centre.....or go for a swim for a fiver!
Hope this helps!