When you start researching your family tree, you have to expect to find the occasional scoundrel in your genetic background. But Penny Deverill wasn't prepared for the tale of murder most foul that she was to stumble upon on her first day of research.

The decision to trace her ancestors, back in 1981, would lead her on a 28-year journey to uncover a torrid tale that caused turmoil in her family almost 200 years ago.

"When I decided to start researching my family tree, back in 1981, it wasn't as popular as it is today," explains Penny, a hospital clerk from Patchway.

"It was long before TV shows like Who Do You Think You Are? would make it a popular hobby. I just thought I'd like to know more about my family.

"I went down to the records office, which was at the Council House in those days, and started to look through the parish burial records for Oldland and Bitton – which I knew was where my family, the Worlocks, had lived for generations.

"But after just a few minutes I came across the record of the death of a 40-year-old Thomas Worlock in 1820, and it gave the cause of death as 'poison'.

"I thought this was a bit strange, but then I turned the page and saw the burial record for a Rebecca Worlock, aged 36, whose cause of death was recorded as 'hanging at Gloucester'.

"It didn't take me long to start putting two and two together. I quickly realised that Rebecca must be Thomas' wife, and it was looking like she had murdered him and been hanged for it.

"In those days, the records office was the sort of place where you had to remain totally silent, but I could hardly contain my excitement at stumbling upon such an interesting bit of family history so early in my search.

"People ask me if I find it creepy to think of a murder taking place in my family, but I always think if your forefathers had all been goody-goodies, you'd never be able to find out any information about them at all."

Over the past 28 years, Penny has worked tirelessly to uncover every possible detail about the case, and the result is a book, Two Penn'oth of Poison – The Story of a Georgian Murder, which Penny will launch at Kingswood Heritage Museum on October 11.

"I have to admit, my family thinks I've become a little bit obsessed by the murder," Penny laughs. "At Christmas, I always get books about executions and hangmen now, because they know I'm fascinated by the history of capital punishment.

"I've even been to Gloucester Jail and stood in the very spot where Rebecca would have been hanged."

By tracking down a range of records, including official reports from coroner's proceedings, transcripts from Rebecca's trial, and newspaper cuttings from the period, Penny and her husband Chris have been able to piece together the full grisly story.

"Rebecca was the daughter of a mill owner in Bitton, while Thomas was the son of the local butcher," Penny explains. "As far as we can tell, it seems she may have married swiftly to avoid an arranged marriage, but she went on to regret marrying beneath herself.

"It turned out that Thomas was a drunkard, and was prone to violent outbursts."

The couple had three children, but after 13 years, it seems that Rebecca had had enough of her husband.

"It came out in court that a few months before the murder, a neighbour overheard Rebecca threatening that she would 'soon put Thomas out of this world'.

"We also know from the court reports that she went to an apothecary in Kingswood Hill, to apparently buy some rat poison, because she 'wanted to get rid of a rat'.

"She wanted arsenic, but the apothecary wouldn't sell her poison on her own – this was a measure they used to take to try to stop wives murdering their husbands.

"But Rebecca simply went outside and offered a stranger threepence if she would accompany her while she bought the poison.

"Apparently when she came back out of the apothecary with the arsenic, she had said to the girl: 'I can tell you the truth, because you don't know who I am, but I have a hell of a man around me who has always plagued me, and this will rid me of him'.

"The girl went home and told her mother, but they had no idea who Rebecca was, so were unable to do anything to stop her."

Meanwhile, Rebecca was going about her deadly deed.

"At first, I don't think she intended to kill him," Penny says. "I think she just wanted to be able to make him ill, whenever he was causing trouble.

"The neighbours began to notice that he would always be ill the day after an argument, but Rebecca blamed the beer that he had been drinking.

"Then one day, after a particularly big argument, she clearly decided that she couldn't take any more, and she emptied a whole 'two penn'oth' of the arsenic into his beer.

"Thomas downed the beer, and immediately started to be ill. He staggered outside where he was sick. He collapsed. His trusty whippet licked his face, and the dog promptly keeled over and died.

"But it would take Thomas a week to die. He was treated by two local doctors, but there was nothing they could do to save him.

"During that week, a deathbed will was drawn up. Although he was literate, Thomas had just enough strength to sign the document with an X mark. He left his money – a sum of £50 – to his wife, with the proviso that she remained a widow and didn't remarry.

"But Rebecca wouldn't be able to get her hands on the money, because as soon as Thomas died, neighbours began to point the finger of blame at her."

The jury at the coroner's inquest agreed that Thomas had been poisoned, and Rebecca was arrested and taken to Gloucester Jail.

"The trial took place at Gloucester Assizes, and it lasted for just one day, before she was found guilty. She was hanged three days later at the jail.

"The judge said her body could be taken to the nearby Gloucester Infirmary, for surgeons' dissection – a common practice with the bodies of murderers.

"What was left of her was brought back to Bitton to be buried, at the bequest of the local Oldland vicar, Rev Henry Thomas Ellacombe, who had promised her this final dignity in return for confessing her sins to him in the jail."

The couple's two daughters were sent to orphanages in London, while their son, eight-year-old John, remained in Oldland, where he brought himself up with eight shillings a year from the Poor Law book.

"Thomas, the murder victim, was the brother of my great great great grandfather," Penny explains. "So my next challenge is to try to work back down the family tree from Thomas' line, to see if there are any descendents around today from Thomas and Rebecca's stormy marriage."

● Two Penn'oth of Poison is available now, self-published, priced £7.99. To buy a copy, call Penny on 0117 9493724.

Penny will be signing copies of the book on Sunday, October 11, 2.30pm, at Kingswood Heritage Museum, Tower Road, Warmley.