We were following one of the guys with a barrow as he wheeled it up the corridor to a vaulted chamber lined with the same white tile. There he lifted a hatch in the floor and tipped the slurry down in one practised movement. With a rattle, he seized a bucket of water placed nearby and sluiced down the wheelbarrow and the edges of the hatch. Then he refilled the bucket from a tap mounted in the wall and wheeled his barrow back down the corridor, presumably for more shit shovelling. As he went I saw another barrow wrangler heading towards us with another load of slurry.

When he led me into the next room I thought I knew what I was going to see next.

I was wrong.

I looked up the figures later; your average pig produces over ten times what a human does per kilo body weight and given that these were big pigs, we were talking a shitload of pig shit. Now, not only is that enough to drown in, but it’s also the vilest-smelling animal by-product known to man – which doesn’t endear you to your neighbours. But you can take that slurry and run it through what’s called a horizontal plug flow reactor. Pig shit goes in one end, some seriously good fertilizer comes out the other and you get methane out the top. It also gets rid of the smell, and some farms do it for that reason alone. The thing is, in a cold climate like what we’ve got you have to use most of the methane to maintain a useful operating temperature, which is why this technology has never taken off in Northern Europe. It’s the sort of sustainable low-tech engineering favoured by progressive development NGOs, Greenpeace and middle-aged men in leather-patched tweed jackets.

I was expecting something simple.

What I got was a ten-metre wall of brass pipework festooned with dials and gauges and stop valves. Two older men in moleskin trousers, white shirts and sleeveless leather jerkins were shading their pale faces while they worked two banks of brake levers, the kind I associate with old-fashioned railway signal boxes. A whistle blew, one of an ascending bank mounted near the centre of the contraption, and one of the engineers stepped smartly over to a row of gauges. There he brushed his fingers around the face of the dial – there was no glass – before calmly pulling two levers in quick succession and turning a valve wheel a quarter turn to the left. The whistle stopped.

My industrial chemistry had been leaking out of my head for over seven years, but enough of the basics remained for me to spot a cracking plant – even one that had dropped out of a Jules Verne novel. The Quiet People were refining their pig-generated hydrocarbons on an industrial scale.

And that was when I realised that Tyburn was wrong.