'Derek the Weatherman’ hopes it will be warm when he takes part in the World Bog Snorkelling Championships in mid Wales. It’s at the end of his walk that follows part of the trail along the Heart of Wales Railway, starting in Llandovery. On his way he takes part in a tug-of-war with young farmers and has afternoon tea under and impressive railway viaduct. Matthew Bassett joins cyclists in Llandovery and wildlife expert Lizzie Daly goes in search of pied flycatchers in rare Atlantic oak woodland.