In the middle of winter, with 17 hours of darkness, 500 competitors head north of Inverness for one of the world's toughest mountain bike races - the Strathpuffer 24-hour marathon. Conditions are testing, with ice and mud pushing the competitors and their bikes to the very limit.

It's fast, it's furious and it's brutal - the Carnethy 5 started over 40 years ago and is the first hill race of the season, when the weather can vary from brilliant sunshine to deep snow. This year 600 hardy competitors battle it out over the Pentland hills just south of Edinburgh.