Growing up, TV presenter, actor and Britain’s Got Talent judge Amanda Holden would often hear stories of “a French connection” in her family. Her middle name, Louise, is a family name, the female version of Louis. But are such tales of French lineage true, or as authentic as a childhood picture of Amanda dressed in a stripy shirt and with a string of onions around her neck?

To discover more, Amanda first goes to see her mother, Judy, whose own research reaches back to Amanda’s 5x great-grandfather, Collin Thomas, who was born in Cornwall c1790. Collin married a French woman, but how did he meet her?

It’s a story that begins in unpromising fashion as Amanda learns that her forebear was imprisoned after joining the Royal Marines. As an apprentice to a shoemaker, he wasn’t free to take the 'King’s shilling' and was sentenced to 12 months of hard labour.

The next time he appears in the records is as a soldier in Wellington’s army during the Peninsular War (1807-14). In May 1814, Collin’s regiment, the 51st Light Infantry, arrived in Bordeaux, which had turned against Napoleon.

Stationed in nearby Caudrot, Collin “deserted for love” and turned his back on getting a military pension to marry Radegonde Charbonnel. They had 11 children together, the first legitimised by their wedding.

It’s a story that appeals to the romantic in Amanda. “What a gorgeous man,” she says. She’s possibly even happier to discover Radegonde was from a winemaking family, who owned a vineyard at nearby Castouret. Amanda is able to visit and, with a glass of the local red, toast her “rulebreaker” ancestor.

Next, Amanda wants to learn more about her paternal grandfather, Frank Holden senior, a psychiatric nurse who committed suicide in his 70s. Because of her parents’ divorce, she knows little about this side of her family, but her uncle George tells her that Frank was “shipwrecked”. How and why?

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As a psychiatric nurse in the Royal Army Medical Corps, Frank was sent to France in September 1939. However, he didn’t escape at Dunkirk. Instead, he was among soldiers who gathered at the port of Saint Nazaire, Brittany in June 1940. Here, he was ferried out to RMS Lancastria, a cruise liner used as a troop carrier.

The Lancastria never made it home. Bombed by German planes, it sank with the loss of at least 4,000 men, the worst loss in British maritime history. An evocative picture shows an oil-covered Frank serving tea to other survivors following the disaster.

Amanda goes out to the wreck site in the company of the youngest survivor, Jacqueline Tanner, who was just two years old at the time. “I was being held, apparently, in my father’s teeth,” Jacqueline says. As to why such a disaster isn’t better known, it’s because Churchill suppressed the news.

Amanda hopes her research will help here. Finally, reflecting on her grandfather’s story, she can’t help but think about the “sad irony” that Frank looked after so many others, yet “wasn’t able to save himself”.