Mrs Filkin, who takes over in February as Parliamentary Commissioner for Standards, is said by colleagues to be an extremely tough operator.
Results of her inquiry into whether Mr Mandelson should have declared a cheap loan from Geoffrey Robinson could easily be critical of Mr Mandelson.
After more than five years dealing with stroppy tax men and customs officials as an independent adjudicator on complaints against them, and for the Contributions Agency, she is not expected to pull her punches.
"I am used to people who dupe themselves, and I am used to quite a number of people who think they can dupe me. That does not mean there may not be some people who have duped me. But I have seen quite a lot of the liars," she told The Independent in an interview before the Mandelson story broke.
Mrs Filkin, 58, has invited members of the public to write to her about MPs' probity.
And there are hints that she may dig rather harder than Sir Gordon when it comes to investigating complaints. In her past role she says, that tenacity has paid off: "I have left no stone unturned and sometimes we have got to the bottom of things because we have gone the extra mile and done an extra bit of investigation."
In pursuit of the truth about complaints, she and her aides have gone out to look at the places where incidents took place, they have trawled through bank accounts and have sought independent corroboration of the stories they have been told. Of course, she adds, most MPs are decent individuals who do their best, and she has no desire to cause unnecessary anxiety to them. The post of Parliamentary Commissioner for Standards was set up in 1995 after a recommendation by the Committee on Standards in Public Life, then headed by Lord Nolan. Lord Nolan found public confidence in MPs at a dangerously low level.
Mrs Filkin, who is married with three daughters and who formerly ran the London Docklands Development Corporation, was head-hunted for her new post. She argues that bar-room gossip might dismiss our elected representatives as a shady lot, but if pushed most people believe most MPs have their good points.
She hints that the House of Commons is a better place since the last general election, when the public showed it was tired of sleaze.
"The public made their views very clear on some of those people. They are not in Parliament. I have no doubt that has had an effect on MPs now - these things are very salutary," she says.
Already, snippets have appeared, hinting that she may be a left-winger because she was once associated with the Charter 88 civil liberties groups. On the other hand, there have also been murmurings about her directorships. "If that's the price of a free press I am delighted to have it. Even if it is hard going and I have to be seriously unpopular, I feel it is a job worth doing," she says.Reuse content