The couple, holding hands, walk down a wooden dock by the water's edge, after spending an evening in a bar on the island of St John in the US Virgin Islands.

They are seen boarding a dinghy and motoring away into the darkness of the Caribbean Sea, headed for their luxury yacht, anchored in the next bay.

This CCTV footage - never made public before and released exclusively now to the BBC - contains the last-recorded sighting of British woman Sarm Heslop in 2021.

Some six hours later, she would be reported missing, feared lost at sea.

Ryan Bane, her boyfriend at the time, maintains she most likely fell overboard, or drowned while swimming, while he was sleeping. He has never been formally questioned by the police.

Despite a massive search operation, Sarm's body has never been found and her disappearance remains a mystery.

To try to establish the truth, I spent a month in the US Virgin Islands investigating the case for a BBC Three documentary - speaking to the police who led the investigation, the coastguard and possible witnesses.

Described by her friends as a "free spirit", former flight attendant Sarm, originally from Southampton, left the UK in 2019 to sail across the Atlantic with friends on a small boat.

"She was always up for a new challenge. Totally run-of-the-mill for Sarm to just be doing something bonkers," one of her friends Zan tells me.

Accompanied by friend Kate and Kate's boyfriend, Sarm headed for the Caribbean - visiting St Lucia, Guadeloupe, Dominica and Martinique.

Seven months into the trip, Sarm met Ryan Bane, 49, an American boat captain who chartered his £500,000 catamaran - Siren Song - to wealthy tourists.

They dated, Sarm stayed on Mr Bane's yacht, and - after three months - the relationship turned serious. Sarm then decided to work as a chef on the yacht and, on 7 March, 2021, completed her first charter.

That night, she disappeared, leaving behind her passport, phone and money. She was 41.

While Sarm's friends and family have tried to keep her story in the media spotlight, and enlisted an investigator to help, her mum Brenda says she accepts her daughter is dead.

But the hope they will establish what happened, she says, is what keeps them going.

"We still haven't been able to grieve properly. We all deserve to know what happened to her and to bring her home. It's just so, so unfair," says Brenda.

One key piece of evidence I wanted to see was CCTV showing the last recorded sighting of Sarm, which had never been made public by police.

So, when the islands' chief of police, Steven Phillip, handed it to me, I was surprised.

"We're at a dead end," he admitted. "If anybody........

© BBC