My wife knows I've lost it, but I took her along with me when we did that run.
If you are not used to (very) long drives, make sure you take plenty of breaks, and leave plenty of time – the roads get a lot slower (and a lot more dangerous) north of Glasgow / Edinburgh.
If you are coming back on the return service two tips –
There is a very good fish restaurant in Mallaig, not far from the station. Highly recommended from a satisfied customer.
If you go out of Fort William station, into the estate with Morrisons (I think) and head in a direction to keep the railway on your right, you will pass the "shed" area, where the loco will probably be being put to bed for the night. We walked it, but you could do at least part by car.
These days we live in Strathspey – so Fort William is a day trip!
Martin