From their web site:

"This is the premier UK event to test teamwork, self-reliance, endurance, outdoor and navigational skills. The reputation of the event is worldwide and every year we have entrants from between 12-14 countries.

The ethos of the event is to be totally self-reliant, in the wilds, carrying all equipment, no outside support and without GPS or cell phone. Your first source of help is your partner and if you retire you are responsible for getting yourselves back to base. Any teams not arriving at the overnight camp are assumed to have camped safely. In these days of 24/7 contact and total support this is an event to test and indeed help develop teamwork and self-reliance."

The Sock