Celebrating their end-of-year exams, the students sat in a circle in the park, laughing and singing. Not far away, I crouched over a ditch, hoping to glimpse one of Britain’s most endangered animals. Ever since our son has been at university here, I’ve often walked along this wide channel of deep, duckweed-green water, praying for a glimpse of a water vole, but he’s graduating soon and I still haven’t succeeded. Today was my last chance. On the other side of the ditch, a round hole peeped out just above the water – the front door of a water vole’s burrow.