Matt Baker and Ellie Harrison are on the border of England and Wales to celebrate the 50th anniversary of Offa's Dyke Path and to explore the fascinating history and treasures that lie along this 1,200-year-old man-made monument. Matt kicks off conservation efforts to help to preserve the dyke and the surrounding landscape for future generations to enjoy, while Ellie abseils to measure and record one of the UK's rarest trees. She also discovers the engineering skills of nature's very own master builders. Adam Henson has got the experts in to calculate the carbon footprint of his farm, and in its 30th year, it's decision time in Countryfile's photographic competition. Which of the thousands of entries will make it into the top 12?