Sir Mark with Lady Thatcher at the funeral of Sir Denis
The relationship between Sir Mark Thatcher and his mother could almost be taken from an Evelyn Waugh novel.
It is an affinity characterised by an "excess of zeal over judgement", according to the former prime minister's biographer Charles Moore.
Sir Mark is the "prodigal son" who keeps on returning. And Baroness Thatcher is there to welcome him with open arms - no matter what scrape he gets into.
He, perhaps more than anyone has seen the Iron Lady's softer, maternal side.
Sir Mark and his twin sister Carol were delivered by Caesarean section in August 1953 while their father watched cricket at the Oval.
Mr Moore told BBC Radio 4's Today programme: "Of the twins, Mark is closer to his mother than Carol, and in that respect
it has made things more difficult because he has been more closely associated
"Carol made her own career as a journalist and won respect for that.
"The counterpoint that doesn't come out so much and does need to be said, I
think, is that Mark is a very devoted son.
"He looks after his mother, he cares for her very much. You could say that
the love each feels for the other, the only thing that is wrong with it is that
it is so strong that sometimes maybe the zeal exceeds the judgment."
Nick-named "Thickie" at Harrow, Sir Mark was never an academic success and his early unsuccessful forays in to the flashy world of motor sports were met with howls of derision by Britain's media.
When he famously went missing in the Sahara during the Paris-Dakar rally it was the first time Lady Thatcher was seen crying in public.
A measure of the depth of a mother's sheer devotion to her son was exposed.
And she was proud too of his talent as a salesman - though perhaps a somewhat colourful one.
"Mark could sell snow to the Eskimos or sand to the Arabs," she once boasted.
But to the then prime minister's press secretary Sir Bernard Ingham he was a liability pure and simple.
When allegations of a conflict of interest arose about his role in a deal to build a university in Oman after an official prime ministerial visit, Lady Thatcher's closest allies urged her to send him abroad. He soon went to Texas.
Some believe Lady Thatcher's devotion to her son is partly explained by her sense of guilt that high office stopped her being a "normal mother".
She entered Parliament six years after the twins were born and her private letters show a real concern for how they are both doing at boarding school.
But the pressure of the Thatcher premiership perhaps fell more heavily on Sir Mark's shoulders than his sister's as he actually lived in Downing Street between 1980 and 1984.
Her affection for him may have been reflected by her desire for her husband to be made a baronet.
That move gave high honour to Sir Denis, but it also produced a title for Mark to inherit.
South Africa visits
Following the death of husband Sir Denis, it is Sir Mark who looks after her and oversaw her convalescence in South Africa.
Indeed, Carol Thatcher's prime concern on hearing news of her brother's arrest appeared to be for the effect it might have on her mother.
She said: "My real concern is actually for my mother. I don't know her reaction and I
care about her."
Charles Moore added: "She is a fairly old and
fairly frail lady, and she is also since last year a widow. And one of the
things that was helping her old age a lot was the capacity to rest more and to
stay with Mark in South Africa and to see her grandchildren."