Sir Roddy, 61, says he would love to present a TV series giving the over-60s age group advice about how to garden better. However, he finds that TV bosses are only interested in appealing to young people.
The baronet and garden designer, who came to fame thanks to his relationship with Princess Margaret in the 1970s, says: "There is one TV programme that I would be brilliant at — gardening when you get older. I have thought a great deal about it. But I think British TV doesn't appear — judging from its choice of presenters — to want (that sort of) knowledge.
"They want so-called 'celebrities' — I hate the term — who are prepared to jump up and down and talk bollocks and are only chosen if they are pretty. But as I consider myself to be the most talented gardening presenter in England, it seems an awful waste not to employ me. I know commissioners are cutting back but gardening is becoming ever more relevant now that the fashion is for grow-your-own vegetables."
Sir Roddy has recently moved to a house in South Warwickshire, where he has planted a vegetable garden. He says: "I can't tell you the pleasure of the taste. The stuff that is grown hydroponically - you might as well eat cardboard if you want nutrients."
He is patron of the Southport Flower Show, held on 20-23 August. He says: "I'm in my 12th year as patron at Southport. I love it. I love the northern people. I would far rather be in a dark cellar with people I love than on the best sun-soaked island with people I loathe."
Sir Roddy will be answering gardening questions on the bandstand at Southport with David Bellamy.
The South Warwickshire-based designer to the rich and famous has arranged for old friend and I'm a Celebrity ... Get Me Out of Here! winner Christopher Biggins to open the garden show this year, following Frank Bruno, Thora Hird, Rolf Harris and Alan Titchmarsh, who have unveiled the popular 80-year-old event in the past.
Sir Roddy, who inherited the baronetcy after his brother Sir Dai died earlier this year, says: "I first met Biggins on the QE2 25 years ago. He's a popular figure and a very nice man, and he will add spark to proceedings. I just have a lovely time as patron going round talking to as many exhibitors as a I can. That's important because the horticulture trade has not been enjoying itself much in recent years so it's interesting to hear what they have to say. Things have picked up a bit after not being very good last year. But it will always be natural for (people) to grow things."
He believes gardening, "like everything else in England", is struggling at the moment and admits to a downturn in projects coming in for his garden design business. He says the lack of show garden sponsors at the RHS Chelsea Flower Show this year reflected the economy, which he "can't see getting any better as it's all so unknown".
Sir Roddy is discreet about who his clients are but it is safe to assume they are from the higher echeleons of society. "They would hate me to mention their names. But it's mainly in England now, though I used to do a lot in Germany and Austria and the West Indies" he says.
He has no set style. "My style is dictated by the whims of the client and the property. You can't just specialise in one style. But it's lovely when people say they want Japanese — the research is fascinating."
His father was Sir Harry, an Olympic gold medallist in showjumping in 1952, and he was educated at Shrewsbury School and then Merrist Wood College. In the early 1970s he lived in a commune and was to grow food for a proposed cafe; this helped start his gardening career.
Sir Roddy says Merrist Wood was "fantastic" and "the best thing I ever did". He explains that stepping into a garden design career after his long affair with Princess Margaret ended in 1980 was "not particularly difficult".
"I'm lucky that I've always had work," he explains. "The largest problem I have is that people think I demand a large fee, which is not true. Because I'm titled it tends to put people off - they think they can't afford me."
But he says the baronetcy offers "no perks" and was created as a money-making scheme by James I in 1611.
He has no plans for new gardening books (of which he has written seven) but is working on a lecture tour of the US for 2010. His current project is his garden at home in South Warwickshire, in "proper country - the last pocket left of what the country used to be". He aims to "earn a living and keep my head above water like everyone else. I have to work. If I didn't I'd die of boredom."
As the "second son of a second son", Sir Roddy says he has "no huge house or rolling acres" and needs to work.
He says the advantage of his title is that it does mean something to the wider society: "And if that means I can be employed as a voice of horticulture then I would love that."
1976-77 National Certificate in Horticulture at Merrist Wood College
1970s to date Garden writer and lecturer
1977 to date Garden designer
1990s TV gardening presenter
1997 to date Patron, Southport Flower Show
2009 Succeeds to baronetcy
Published Town Gardens (1981), Beautiful Backyards (1986), Water Gardens: The Connoisseur's Choice (1987), Elegance & Eccentricity (1989), Growing Gifts (1991), Grow It Yourself: Gardening with a Physical Disability (1993) Roddy Llewellyn's Gardening Year (1997)