Sir Thomas has audited MPs' second-home expense claims made since 2004
The MPs' expenses audit has been conducted by lawyer and former civil servant Sir Thomas Legg.
Sir Thomas was permanent secretary in the Lord Chancellor's Department from 1989 to 1998, having joined in 1962.
In the recent controversy over his MPs' expenses audit, some have accused him of simply running with public opinion, of reflecting what the newspapers say.
But Labour MP Tony Wright, who has known Sir Thomas since his civil service days, disagrees: "You'd be hard-pushed to find a man less likely to do that than Tom Legg. Whatever else he is, he's not a rabid populist.
"He's his own man, and he'll do a job that's got to be done, in his own way," Tony Wright told BBC Radio 4's Profile programme.
Sir Thomas was trained from a young age to challenge institutions.
Born in 1935, his father was a noted documentary film maker, who educated his son partly in New York, and partly at Frensham Heights, a school in Surrey with an unconventional style and mix of pupils.
Its headmaster Andrew Fisher says that "there was an interesting tradition in the school in those days, where the students would meet with the headmaster and would discuss and propose changes for the school".
The students were encouraged to question their superiors. Andrew Fisher adds that this strengthened "skills of debate and public speaking as well as having a strong opinion about how the school was developing or being managed".
This public service, this duty to the public, is one that he observed
Legal commentator Joshua Rozenberg
Sir Thomas then had a long career as a civil servant, during which he appears to have developed a strong belief in the highest standards of behaviour in public life.
He published an obituary in the Guardian newspaper a few years ago of one of his former colleagues, Hume Boggis-Rolfe, praising him for his rock-like integrity and thrifty habits:
"Gladstonian in his economies great and small, he kept a desk drawer full of old pieces of paper and string for re-use, so that the public funds should not be wasted".
Legal commentator Joshua Rozenberg, who knows Sir Thomas personally and has followed his career for many years, considers this is a very telling obituary.
"I do think that certainly some of these characteristics are ones that Tom Legg himself would follow.
"I've never regarded him as mean, but on the other hand I certainly wouldn't have thought of him as being particularly extravagant.
"But certainly this public service," Rozenberg adds, "this duty to the public, is one that he observed."
The MPs' expenses audit is not the first time Sir Thomas has been asked to look at MPs' finances.
Until recently, he was one of two external auditors of the Members Estimates Audit Committee, which was set up in 2004 to consider matters relating to pay and allowances on behalf of the House of Commons.
And he is no stranger to investigating public affairs.
In 2000 he conducted a parliamentary inquiry into the large overspend on Portcullis House, the office building for MPs opposite the Palace of Westminster.
His report was never published.
But the inquiry gave Sir Thomas an insight into attitudes towards spending in Parliament.
Two years earlier, he had co-headed an inquiry into the arms-to-Sierra Leone affair, finding no evidence of government-approved breaches of a UN arms embargo.
Sir Thomas was appointed an honorary QC in 1990, and has been a consultant at law firm Clifford Chance since 1998.
He has sat on the Audit Commission for several years.
He was called to the Bar in 1960, after graduating from St John's College, Cambridge, where he read history and law.
His national service had been in the Royal Marines from 1953 to 1955.
Past and present chairmanships include Hammersmith Hospitals NHS Trust, Imperial College Healthcare Trust and London Library.
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