All things being equal I'd go for the train/bus combo. Why? Well, the train journey from InverBubbly > Strathcarron is a joy, as is the bus up to Torridon. And the train/bus connection is secured. When you get on the train, be sure that get into the correct carriage. The train splits after a short distance. And take a seat on the right-hand side of the carriage - this way you'll have a mighty view over Beauly Firth.

I do like a period of de-compression before starting the Challenge, and prefer to make my own way. But each to her/his own!

I'm starting from Shiel Bridge this year - first time from that locale - and am training it down to Kyle of Lochalsh and then a short bus trip east simply because the train trip is so fine.

But Gordon and Andy's suggestions re taxi are also excellent.

Thanks for the advice everyone, think I'm going for the train/bus combo.
De compression prior to the challenge is to be recommended! I remember last time spending the first 2 days with dreadful headaches as my body rid itself of the various toxins! This time I'm taking coffee and whiskey so will hopefully avoid a cold turkey this time.