This week, we’ve been tramping around the vast Highlands and Islands of Scotland, famous for its monster lochs. Loch Ness alone holds more water than all the lakes in England and Wales combined. We may well glimpse Nessie this week, but we’ll definitely see these five keen walkers who want to bag the big prize. Today, it’s the turn of 26-year-old Katrina, the tattoo shop manager from Glasgow. She’s been hiding an aquatic secret from the others. She has a part-time job as a mermaid, performing in aquariums. Katrina's walk starts and finishes on the shores of Loch Long - a haven for dolphins and porpoises, with some of the clearest water in Britain. Will she manage to persuade the walkers in for a dip? And can she pull out all the stops to take the lead?