You're welcome, Nicola.
I'm assuming you're taking the Kilchoan ferry to Mallaig since it's a shorter drive? If so, here is something you might want to know.
Ferry terminals have numbered spaces for cars to get in, and that’s how they know the order they will proceed onto the ferry. For example, number one goes first, and so on. If you are taking the Kilchoan ferry, which is a VERY small ferry terminal right at the end of the dock in Tobermory, I would suggest taking your car down there early, parking in the lowest number you can park in, then just lock it up and go have breakfast someplace. :-) When I was there, I got up around 6-ish am (I wanted to be on the first ferry of the day), and I managed to park in the number one slot. Since it is a show up and go ferry, it’s better to be at the head of the line to make sure there is space for you – especially if you have a car.
After locking it up, I walked back to the guest house, had breakfast, paid my bill, and walked back to the ferry. By this point, a crowd was starting to gather. When I walked up to my car so i could put my small backpack in it, a woman came up to me and asked me what I had to do to get that spot. I told her that you had to be there at 6 am. She laughed and said she wanted to be here then, but her husband refused to get up that early.
Anyway, you may not need to be there that early, but I wanted to, because as Stuart has noted, you just never know with the ferries and it's always better to be too early than too late. But by the same token, even if you don't want to leave that early, you could still get your car down there right after the previous ferry leaves and snag an early spot for the next ferry.