From the time he failed the entrance examination to Eton, Roddy Llewellyn's life had been marked with no great success until he met Princess Margaret, 16 years older than he, and sister to the queen.

His liaison with Margaret, highly publicized in the British press turned his fortunes around especially now that there is some hint that the crown is understanding, if not approving of the relationship.

After his failure at Eton - all the more gailing because older brother Dai had sailed through - Llewellyn settled for Shrewsbury. This was painful because Shrewsbury is several notches down the social ladder. Singing in the choir helped a little.

Llewellyn, friends say, wanted to please his father. Bluff, overhearing, athletic Col. Sir Henry Llewellyn, but the youngest just did not like fox hunting.

"I am not practicularly interested in killing small animals," he said.

So he did the best thing and went into his father's business, the big brewery of Whitebread where Col. Sir Harry is president of the Wales concern.

Whitebread sent Llewellyn to Sheffield, a steel town, and he hated it.

So he was sent to London. He hated Whitebread but he liked the nightlife. He shared an apartment with Nicky Haslam, an interior decorator. Llewellyn took to wearing a gold earring.

He work briefly as an assistant herald at the College of Arms. He liked tracing ancestries. But no matter how he traced, he could not overcome the fact that older brother Dai and not Roddy would inherit uncle Rhys' baronetcy.

He drifted into landscape gardening.As his mother once said, Roddy "loves flowers and is looking after my sister's camellias."

Col. Sir Harry was not amused. Dai may have become notorious among London's nighclub denizens but at least he loved fox hunting almost as much as girls.

Five years ago, Roddy Llewellyn was treated for an overdose of valium. His friend s agreed that it was a shame because the boy - everybody always refers to Roddy as a boy, even now when he is 31 - was and is charming.

He was amusing, vulnerable, graceful, the perfect extra man for a country house weekend. And so Roddy was invited as the extra man one weekend for a woman at loose ends, Princess Margaret.

Those who were there say the pair hit it off at once. Margaret had not yet formally separated from her husband, Lord Snowdon, and she had a boy and a girl of her own. Roddy has an uncanny resemblance to Snowdon.

Soon they were crooning old songs together. "Me and My Gal" was aparticular favorite. Llewellyn has recalled that the princess has the louder vouice.

She began calling him "my darling angel."Among close frieds, he would call her "P.M." (Princess Margaret: or "Brenda," (her childhood pet name) or simply "darling."

Llewellyn joined some of his nightlife chums in a commune at Wiltshire. He was proud of it and had Margaret stay at the ramshackle farm several times.

This was serious. "I'm terrified about the world food shortages," Roddy said. "Vegetables could quite easily become tomorrow's currency. I'm also producing Surrendell Wine from dandelions."

But the drought killed the days of wine and dandelions. The commune broke up. Roddy drifted away and colected welfare checks.

A friend introduced him to Petula Clark, a singer, and the pair did duet on french television. Roddy was launched on another career. Clark's husband will soon bring out an album of songs by Roddy-Llewellyn and then the world will hear the voice that enchanted a princess.

Llewellyn does not deceive himself.Martin Walker of the Guardian asked him whether the scandal had helped his new venture.

"Let's be honest," he replied. "It helped. Th - er - fame made it easier."

Llewellyn is exploiting the royal connection in unfashionable Batter-sea. He has an interest in a new nightclub there and most of London's did launching. Llewellyn has pitched in with a bit of the interior decorating, although lanscape gardening appeals to him more, his mother has said.

Col. Harry got a knighthood last year, so he must be a bit more pleased with his boy, who, he had earlier been quoted as indicating, was blocking with his father's social progress. When Roddy, vacationing with Margaret on a Caribbean island recently came down with a stomach ailment, the colonel gamely attributed it to his son's strenuous surfing. In fact, it was an ulcer.

Anyway, the colonel can always look at the gold olyampic medal he won in 1952 riding, of course, "Foxhunter."

Roddy llewellyn isn't the medal winning type. But he has somehow acquired a prize unique in this realm.