The picture emerging of Gareth Williams could be that of a textbook spy: athletic, intelligent and private.

Detectives investigating the murder of a British spy were picking over his private life yesterday for clues that could identify his killer.

The decomposing body of codes expert Gareth Williams, 30, was found stuffed into a bag in the bath of his London government flat.

He was days from completing a one-year secondment to the headquarters of MI6 from his job at national “listening post” GCHQ in Cheltenham, Gloucestershire.

Originally from Wales, he studied at Cambridge University – one of the traditional recruiting grounds for the British secret services.

He excelled at maths, embarking on a postgraduate certificate at St Catharine’s college in 2000.

But he dropped out after a year and began a career working for the secret services.

Relative William Hughes said he worked for GCHQ, the government’s so-called listening post, for “many years”.

But he kept quiet regarding the actual nature of his work.

“He would never talk about it and it felt rude to ask,” Mr Hughes told the BBC.

Jenny Elliott, his former landlady in Cheltenham, described Mr Williams as “a lovely guy”. She added that during his time living in Cheltenham he was often sent overseas, spending much of this time in America.

Residents close to his London address said Mr Williams was “extremely friendly”. A keen cyclist, Mr Williams competed in races in Cheltenham, and was described by those who knew him as extremely fit.

A fellow member of the Cheltenham & Country Cycle Club told The Times: “He turned up at events, rode very well and then went on his way. He never took part in any of our more social rides.”

One London neighbour also attested to his private nature. Secretary Laura Houghton, 30, said Mr William’s windows were always shut with the curtains drawn. “I could never tell if anyone was in,” she added.

Believed to be single, his former landlady said Mr Williams rarely entertained and did not have a wide circle of friends.