The family of a British spy whose body was found padlocked inside a bag believe secret agents versed in the "dark arts" tried to cover up his mysterious death, lawyers told a coroner on Friday.

The decomposing remains of Gareth Williams, 30, were found on August 23, 2010, in a North Face holdall bag in the bath at his London home, near the headquarters of external intelligence service MI6 where he worked.

Anthony O'Toole, a lawyer for the relatives of Williams, told a coroner that the family believe someone else was either present when he died, or broke into his home afterwards to destroy evidence.

"The impression of the family is that the unknown third party was a member of some agency specialising in the dark arts of the secret services -- or evidence has been removed post-mortem by experts in the dark arts," he said.

Coroner Fiona Wilcox is holding a review of evidence ahead of a full inquest into Williams's death. Inquests are held in England to examine sudden or unexplained deaths.

Wilcox said she wanted to see a practical demonstration of how Williams could have got into the bag and locked it by himself, given that one suggestion is that he did so as part of a sadomasochistic sex ritual.

She said the issue of whether Williams was alive inside the bag and had locked it himself "was at the very heart of this inquiry".

The coroner also called for an explanation after Scotland Yard revealed that two key lines of inquiry pursued by police for months had both been in error.

Forensic teams flagged up a spot of DNA on the dead man's hand in 2010 for investigation, but realised two weeks ago that it belonged to a crime scene scientist, the force told the coroner.

A Mediterranean couple that police said they urgently wanted to speak to in connection with the death were also found to be irrelevant to the inquiry, they said.

Williams was described by friends as a shy mathematics genius and keen cyclist.

He was just days from completing a one-year secondment to MI6 from his job at GCHQ, Britain's electronic "listening post" which monitors communications for intelligence purposes, located in Cheltenham.