BORN into the east end of Glasgow in 1948, Lulu might have suspected a shake of her family tree would reveal tales that were less Enid Blyton and more Brothers Grimm.

Even so, the Scots singer was not prepared for what emerged when she became the latest subject of the BBC’s Who Do You Think You Are?

In an episode to be shown this week, Lulu discovers that her grandfather was a character who could have walked from the pages of No Mean City, the 1920s-set novel that became a byword for a Glasgow scarred by razor gangs.

“I had a criminal for a grandfather,” says Lulu after finding out that her mother’s father, Hugh Cairns, served time in a variety of prisons, including Barlinnie, from the age of 16.

Lulu’s aim in taking part in the programme had been to find out why her mother, Elizabeth, was the only one out of seven children born to Cairns and his wife Helen to be given away.

“The story of my mother is a big secret,” says the singer. Elizabeth’s discovery that she had been fostered as a baby “played havoc with her, with her state of mind”. It was a subject young Marie Lawrie and her three siblings learned not to talk about.

The BBC show has gained a reputation for reducing its subjects to tears as ghosts from the past rise from the archives. Former Newsnight presenter Jeremy Paxman wept on learning that his Glaswegian great-great-grandmother had been turned down for poor relief because she had an illegitimate child, a decision that forced the widow even further into poverty.

Lulu, 68, proves no exception to the Who Do You Think You Are? rule as she heads from her home in London for a quest that takes her from Springburn railway depot, where her grandfather once worked as a labourer, to the Winter Gardens in Glasgow Green, the Mitchell Library and an Orange Lodge in the east end.

With Cairns a Catholic and Kennedy a Protestant, the pair’s relationship defied the city’s sectarian divide. Kennedy’s mother was also a leading figure in the Orange Lodge, making the couple’s union more problematic still.

“In those days for a Catholic and a Protestant to come together was almost impossible,” says Lulu.

Cairns’ family paid for him to emigrate to America. But, missing Kennedy, he stayed only a few months in Boston before working his passage back to Glasgow. It was one of several unsuccessful attempts to split the couple.

“It’s clear there was a very passionate relationship but I have to say it doesn’t look like their future is bright and rosy,” says Lulu as more information emerges.

Those fears are proved correct as the twice-married grandmother discovers what happened to her own mother. In time, Elizabeth, given away to foster carers not long after her birth, would end up with McDonald family, where she stayed for good. Records show she was happy and well cared for.

“How unbelievable they were,” says Lulu of the family who took her mother in. “They are the kindest, most loving, wonderful people.”

Cairns and Kennedy’s other children would not be so lucky.

Lulu thought she had never met her grandfather, but a photo shows the two together at a family wedding when she was very young. Cairns has a deep scar running from lip to ear. “A Glasgow grin,” says Lulu.

Some of Cairns’ arrests took place during the city’s summer marching season. On one occasion a large fine was paid on the day of his conviction, leading to speculation that he was part of one of the gangs of the time. In total, Cairns was in prison ten times in ten years.

“I don’t know anybody who was in and out of jail like that and I feel I came from a tough part of Glasgow,” says Lulu.

“Thinking about my grandparents’ life makes me realise how awful their struggle was. He had to be in a gang or be nothing. Not to negate the fact he made choices that were not smart. Her, I think she married a wrong ‘un.”

Who Do You Think You Are?, BBC1, Thursday, 8pm.