It could be a scene from a photo shoot for Country Life. A log fire crackles in the baronial hall of Beckley Park, Oxfordshire, a grand Tudor house whose ancient walls are hung with tapestries. Large vases cascade with fresh flowers. Outside, there is not one, but three moats.

The lady of the manor, Amanda, Lady Neidpath, is dressed in velvet and tweed, with not a scrap of make-up on her face. Her appearance is arresting, in a shabby-chic manner. Only when she opens her mouth to speak is this deeply traditional scene somewhat shattered.

For this morning we are talking about her fondness for illegal drugs. In the world of psychoactive substances, she is an acknowledged expert: her charitable trust and think tank, the Beckley Foundation, is devoted to the investigation of consciousness and the aim of reforming drug policy. Which all makes perfect sense when I discover she has spent a lifetime dabbling in mind-altering substances. As she tells me enthusiastically: “I have always considered myself my own best laboratory.”

This month her foundation published a 226-page document – the global cannabis commission – that contains the measured considerations of five internationally respected scientists on the use, prohibition and control of cannabis. The commission’s report, which cost her more than £80,000, was launched with some fanfare at a conference in the House of Lords, with grandees from the United Nations and the European Union in attendance as well as leading scientists, legal advisers and drugs specialists.

Neidpath hopes it will be the definitive guide to all available evidence on the cannabis issue. It doesn’t quite recommend that the drug be legalised, but urges the lifting of criminal convictions for use or possession, and suggests low fines or counselling as an option for countries that prohibit the drug.

“We should have anormal relationship with cannabis,” says Neidpath, who is 65. Unlike some campaigners, she doesn’t think it’s harmless – but she does think it’s a lot less bad for us than tobacco or alcohol. She snorts. “It’s a fallacy to think that after a few puffs of cannabis your child will be lost in a psychotic quagmire.” However, Neidpath points out, much of today’s cannabis has been genetically modified to be up to three times as strong as it should be. “Which needn’t be damaging, if people know how to use it,” she says.

“The natural mixture is being changed, and the antipsychotic element in it is being left out. If cannabis was authorised, it could be properly labelled, and government-controlled.” Rather like the tar content on cigarettes or the percentage of alcohol in wine? “Quite.”

Can she see this ever becoming a reality?

“Society has to grasp the nettle. One wants people to make educated choices, as long as they don’t affect other people. It’s a terrible infringement on personal liberties that your cognitive freedom – how you play with your brain in the privacy of your own house – is controlled.” Neidpath is clearly all too accustomed to playing with her own brain, merrily listing the drugs she has taken: “Psychedelic drugs and cannabis. Mushrooms, mescaline, LSD – these have been used since the beginning of humanity.”

It’s quite an impressive-sounding range, but she draws the line at over-the-counter drugs: “I’m more nervous about things which deaden the brain. I hardly ever take painkillers. Do you know that 2,500 people die of painkillers a year, which is a lot more than die of illegal drugs? I’m very puritanical about what I take.”

According to Neidpath, ancient cave paintings demonstrate that even Stone Age man enjoyed a bit of dope. “In cave paintings, the lines they use are the same that people produce when they are on certain substances.” The great thinkers of ancient Greece were almost certainly on drugs too, she thinks. She is not, however, interested in drugs such as heroin or opium – and calls cocaine, which she’s taken “once or twice”, a thoroughly boring substance.

“It’s a greedy drug which brings out the less interesting side of humanity. It’s typical that it’s used in the City. Psychedelic drugs are another matter altogether.”

Has she ever tried taking LSD and writing down what’s going on in her head? “Oh yes. I’ve had periods in my life when I’ve used it to stimulate my brain. I was a painter, and found it stimulated my visual senses amazingly.”

She even had a spell during which she would drop a couple of tabs before playing Go, the Japanese tactical game. “I’d find I would win more games in a row. So your handicap went up. It was quite a good cognitive test.” This is not to suggest, however, that Neidpath is simply some aristocratic hippie who enjoys playing board games while her mind turns chemically inspired somersaults.

During the week, her husband, Lord Neidpath, lives in his own family home – the Jacobean mansion of Stanway in Gloucestershire – while his wife is at Beckley Park, working long hours at her foundation (the couple meet at night and at weekends).

A distant relative of the Hapsburgs and cousin of the Earl of Denbigh, Neidpath was brought up in this house – which featured in the second Harry Potter movie – and has lived here all her life.

Have her two sons, Rock and Cosmo – from an earlier relationship with the writer Joey Mellen – experimented with drugs? “I can’t speak about that because it’s illegal. But more than 50% of young people do, so I would be extremely surprised if my younger son hadn’t,” she says. “And I don’t object to my sons’ friends or whatever taking cannabis. What I do object to is if they mix tobacco with it because I hate them damaging their lungs.”

As she points out, if they were enthusiastic participants, it doesn’t seem to have done either child much harm: Rock got a first-class degree at Oxford and Cosmo has just achieved a double first there in classics, and an invitation to apply for an All Souls fellowship. “He [Cosmo] did give up smoking cannabis six months before his finals,” admits Neidpath, forgetting about her vow of silence on the matter.

Although she seems unconventional, she is in many ways fully within the tradition of the true English liberal. Famously, however, she has gone further than most in her investigations into altered states of consciousness: as a young woman she had a hole drilled in her skull.

Why on earth did she do that? “Trepanning is the most ancient operation in the world,” she explains. “They did it in the Stone Age, and traditional cultures do it in the jungle now. The brain is surrounded by three strong layers of membrane; trepanning removes a piece of bone from the skull so that the membrane inside the brain can expand on the heartbeat.”

Neidpath did her own trepanning with the help of Mellen, who also filmed it being done. Amazingly the experience didn’t put her off. “About 30 years later, I thought probably the hole had closed so I had it done again.”

She points an elegant index finger to her right temple to show me the exact location of her 7mm hole. Afterwards was there any difference in how she felt? “It’s difficult to be in any sense sure of what is what. I observed a change in my dream patterns, and a slight, subtle change. I felt a bit more buoyant.” She believes that through increasing the supply of blood to the brain, trepanation may prove to be a protective measure against dementia.

Why is she so fascinated by the mechanics of the mind? The powerful combination of a liberal upbringing and an early interest in mysticism seems to be the answer. “I hit the beginning of the Sixties with a very open mind,” she says.

There’s certainly no sign of it closing over.

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