When commissioning editors hire "alternative" presenters, hoping to liven- up current affairs slots, they are usually obliged to stick to "sexy" subjects, with "yoof" appeal - anything related to drugs, cruelty to animals or the environment. Thomas'ssubject seems about as unsexy as it gets - the Conditionally Exempt Land and Building Scheme, a 20-year-old Inland Revenue initiative that gives the owners of stately homes and open countryside exemption from inheritance tax in exchange for a public right of access.
But, as he puts it, "it's a story of death, money and politics". Instead of the traditional piece-to-camera, Thomas uses stand-up comedy to investigate the use and abuse of the scheme. What he discovers is that it is almost impossible for the public to visit the houses and grounds involved because no one is allowed to know which sites are tax exempt. If the Inland Revenue were to reveal who and what is on the conditionally exempt list, it would be breaking the principle of taxpayer confidentiality.
"In theory the scheme is a fair deal," Thomas says. "But at the moment it is riddled with stupidity and malpractice. It's worth remembering that many more people visit stately homes every year than attend live theatre performances. You're talking about an enormous amount of people who should be enjoying this scheme. But no one is accountable, and the bottom line is that people are being cheated out of their money."
Many of the tax-exempt owners he tracks down are what he calls "Trotskys in Tweeds" who say they would have no objection to being on a published list. But list or no list, they are also meant to make reasonable efforts to publicise the fact that they are open to the public, and Thomas finds that few do. The people living near Bradley House, the Duke of Somerset's residence, had no idea that they could visit his home. Thomas takes them there in coach-loads, places advertisements in the local press and flies a balloon, emblazoned with the Duke's telephone number, over the area.
This is not the first time he has become embroiled in the intricacies of tax laws - he investigated a similar scheme on his show, The Mark Thomas Comedy Product. Thomas admits to being "obsessive" about the subject, but denies that he is tilting at windmills. The world of heritage, he argues, is chokingly incestuous and self-serving, and it exemplifies how "old power" still rules. "The establishment continues to protect itself," he says. "The idea that the aristocracy is somehow in decline, or has been replaced by the nouveau riche, is rubbish."
But it is not just class war against old power. He is equally at odds with new power: "The most horribly confirming moment was when we realised the Dawn Primarolo and Chris Smith [Financial Secretary and Heritage Secretary] weren't going to talk to us
His mission now is to change the law by making a mockery of it. He is asking the public to help track down as many conditionally exempt sites as possible, with a view to publishing the list himself. The campaign includes a Tip off a Toff hotline, his version of the DSS's Beat A Cheat, and an Amnesty for the Aristocracy.
"Dispatches", Thursday 13 November, 9pm, Channel 4.