Hartsop, Cumbria: I look down and notice a slug-sized, dark brown invertebrate clinging to my leg

A good September day can feel more like true summer than any other time of year, and this sunny, sultry Sunday in the Patterdale valley is an excellent example. The landscape has eased into a settled maturity: the hedgerows are full of dark fruits, the rowans are full of lipstick-red berries, and juvenile sparrowhawks call out from woods of deep, well-aged green. This late-summer lull feels like the equivalent of a piece of music resolving on a satisfying chord, the culmination of everything the year has been building towards.

I am on holiday here with my girlfriend and some of her family and friends, staying above the village of Hartsop, close to Brothers Water. This small, shallow lake is home to rare species such as the schelly (Coregonus stigmaticus) – a relic whitefish endemic to just four Lake District lakes – and a community of bottom-rooted plant species, some of which brush slimily against my legs as I go in for a quick dip. My companions are fazed by the reeds, but I wave away their concerns with the haughty confidence of a seasoned wild swimmer.