Bat conservation is open to everyone, even city nature-phobes

It's International Bat Weekend, an annual celebration of the flying mammals that live in trees or in the crevices of buildings

• Charlie Hearst is a National Bat Monitoring Programme volunteer and member of the London Bat Group

It's getting dark. Thunderstorms are forecast and I'm loitering around a park in Croydon past closing hours, waiting to meet a total stranger. Alarm bells should be ringing if it weren't for our shared interest: bats. This isn't an eccentric date, I'm tagging along to a bat survey.

The stranger is an ecologist for the National Bat Monitoring Programme (NBMP), an annual event in which volunteers across the country survey bat populations. Since 1996, this world-leading project has given us an insight into the long-term population trends of UK bat species - an invaluable resource for conservation and government bodies. While I'm no stranger to bat surveying, this is my first time participating in one as part of the NBMP. I've also taken the opportunity to drag my friend Wayne along for his first bat walk.