You can find happiness is a small piece of corrugated metal. “Do you want these?” I asked my neighbour Tony. He was parking at my place at the time and his trailer held three ragged chunks of the stuff, each about a yard square. “I was going to throw them away,” he said. So I took possession of them and found joy.

I took them out onto our scrap of marsh and placed each one on the ground in a likely-looking spot. And that was the end of the hard work. Now, every time I walk the marsh, I pause for a moment by each chunk – each “tin” in the jargon – to see what lies beneath.

It’s an old dodge used by herpers, naturalists who specialise in reptiles and amphibians. When the sun comes out it creates a micro-climate underneath the tin, and here the reptiles love to bask.

It’s a freebie: they’re getting slightly more warmth for their money than they would in plain sight, so a tin works as a magnet for them. It’s especially good for slow worms, but I suspect the marsh will be too wet.

The vegetation has sprung up around the tins, and the stuff below has gone yellow. Mice enjoy the warmth and shelter, leaving signs of their presence in busy little holes.

But the prize I was looking for from the moment I set down the tins was grass snake. And after several weeks of fruitless checking, I found the first one. Tiny, not much bigger than a bootlace, dark with a bright yellow collar. Exquisite in every tiny detail. Since then, and so far, I have found grass-snakes on two more occasions: all of them about the same size. I’ll be checking again later today.

Snakes are hard to see. They have their being in obscurity. Their shape makes them supreme at the art of sneaking. In the Luangwa Valley in Zambia several species of raptors specialise in snakes: so there must be an awful lot of them about. But I’ve hardly ever seen a snake in all my many visits: they are brilliant at keeping out of sight.

The tins are a way of conjuring them into sight. A tin is a bit like a magician’s top-hat: snakes appear unexpectedly in the manner of a conjuror’s rabbit, though they’re a trifle more alarming.

Grass snakes can get big. I once saw a female swimming across a pond and she was a good six feet. There she was, a snake as long as a bloody hosepipe, calmly crossing the water as if she was queen of the wildest place on earth.

So it’s something to think about as we walk out on our civilised and cultivated island. There, out of our sight, but in the right places all around us, are snakes as long as Maria Sharapova is tall. Britain is still wilder than we know. I wonder if those babies of the marsh will attain such a length. That’ll make me jump when I lift up the tin.