It's a crisp, sunny day and retired church minister Peter Twinn is out with his metal detector unearthing evidence of Roman occupation beneath the bare fields of South Gloucestershire.

"Over there is the Roman temple and just beyond those trees is the villa," he says. "No one knew about this site before I came up here – it's all new discoveries."

As we walk he sweeps the detector back and forth in front of him as it chatters away in a series of clicks, whoops and whistles.

"It's a bit like being out with the Clangers," he adds, smiling. "You have to have the ear to know what it all means."

Twinn bought a second-hand metal detector when he was in his teens and says he's "never looked back". He is now one of thousands involved in the hobby nationwide – a number expected to grow sharply following the announcement last autumn that "detectorist" Terry Herbert had found a huge hoard of Anglo Saxon gold in Staffordshire valued at £3.285 million. Last week, the historian David Starkey launched a campaign to keep the "Staffordshire hoard" in Britain, further adding to the excitement.

All around us chunks of Roman masonry dot the field. Twinn believes that hidden in the soil below is an extensive Roman ritual and domestic site, still largely unknown and undug, which is why the precise location of our walk must remain top secret....