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.

Back at the house, I sit in the sun for a while, chatting and soaking up the view, then go for a shower. But my reflections on the idyllic day are interrupted when I look down and notice a swollen, slug-sized, dark brown invertebrate clinging to the back of my leg, some way above the knee.

On reflection, it seems likely I had picked up a passenger from my swim – the leech Erpobdella testacea, perhaps, which is another species known to inhabit Brothers Water.

Our horror of leeches is out of proportion to the harm they inflict, which is generally none at all; indeed, they are still deployed in some types of surgery for their anticoagulant effects. If I had more presence of mind, I might have captured the creature and returned it to the lake. But in a stab of instinctive panic, I flick it off, then feel a pang of remorse as it swirls into probable oblivion down the plughole.