It feels like a scene from a horror film. You nip jauntily across to the window, open the shutters then recoil in shock before a horrible seething mass of big juicy flies. We’ve known them reduce a room of resilient volunteers to a shivering mass on an innocent September day. We’ve also been called out to help on a snowy Christmas Eve to find country house residents terrorised by a buzzing host. The flies had been roused unseasonably from hibernation by electric heating. Ugh.