Inspired by his 11-year-old daughter Katy, a keen treasure hunter, Wyman is preparing to unveil his own range of scaled-down metal detectors suitable for small hands.

He has spent months testing the device — due to go on sale before Christmas — around his 15th-century manor house in Suffolk, where he has found rare coins and ancient pottery.

Wyman, who will celebrate his 70th birthday in October, said he wants to bring British history alive — as well, he joked, as supplementing his £34 a week national pension. He said most of his £20m fortune is tied up in long-term investments, which occasionally leaves him short of cash.

Since quitting the Rolling Stones in 1993, weary of long tours and bickering, Wyman has built up other interests ranging from his Sticky Fingers hamburger chain to books, including Treasure Island published last year — a young person’s guide to buried artefacts.

“When I was doing that book I met many young people who had found treasure, like a 10- year-old kid who found a medieval sword in a river,” he said.

His own successes include finding 20 Roman brooches and a gold half noble coin from the reign of Edward III, valued at £2,000.

“I’ve been detecting with my oldest daughter, who found a buckle from the 1300s and a miniature toy cannon from the 17th century which is now a collector’s item,” he said. “But after five minutes she’d say, ‘Oh, I can’t do it any more, daddy’, because the detector gets too heavy for her.

“We got her a plastic one for Christmas but it lasted five minutes, so with C-Scope (a detector manufacturer) I knocked up a prototype which works about 80% as well as an adult one. I want to get the price under £100 before we launch it.”

Wyman lives a quiet life near Bury St Edmonds with his actress wife, Suzanne Accosta, and their three daughters, Katy, Jessica, 10, and Matilda, 8.

“I sometimes go out (detecting) with a friend, but mostly on my own because it’s very rural and quiet. It’s about getting away from your hectic life and enjoying fresh air, nature, exercise and British history all at the same time.”

It is a long way from his colourful days with the Stones during the 1960s when, he has boasted, he had up to five girlfriends at a time and slept with more women than either Mick Jagger or Keith Richards.

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