Hi, nice to have you as a friend!
Further to what I just posted, I'd recommend Fort William Car Hire if you're thinking of hiring a car. It's a small family business and they also give you a free sat nav.
Last time I went on the Sleeper I stayed overnight at Corrour - I took the 1737 from Fort William, stayed overnight, then travelled back next morning on the seated Sleeper. I'm not sure if it's open in December though.
I'm going up this Thursday and back Sunday or Monday - will let you know how it goes!
BW
Mark