The following update was added on Thursday 18 November 2010

Mark Andrews appealed against his conviction for causing actual bodily harm. In November 2010 he won his appeal. The appeal judge said he was satisfied that Sgt Andrews did not intend to throw Ms Somerville into the cell and that injuries she suffered "were probably caused by her falling to the floor after letting go of the door frame.

More than two years on, Pamela Somerville is still dealing with the emotional and physical aftermath of her ordeal in a police cell in the quiet Wiltshire town of Melksham.

Somerville, 59, fears she will need an eye operation, and the left side of her face has dropped lower than the right. She still cannot believe that as an innocent and law-abiding citizen she could have been treated so brutally.

The attack happened in July 2008 after Somerville was arrested close to her home in North Colerne, a village between Chippenham and Bath.

Following a "minor row" with her partner, she had started to drive to her daughter's house in London but then pulled into a layby and listened to some music. It was night and she fell asleep.

The next morning she was found by police, who apparently assumed she was drunk. She was later told she had refused to give a breath sample but denies this and was not charged.

Somerville claims a police officer handcuffed her at the side of the road and threw her over the back of a car. She says it was like a scene from a police drama.

She thought there must have been a murder or terrorist incident and she was a suspect. It even went through her mind that she was the victim of a prank.

She was driven to Melksham police station where she met custody sergeant Mark Andrews, a 191cm (6ft 3in) former soldier. She asked him why she was there but he told her to shut up and she was taken to a cell.

When a doctor came to examine her she slipped out but was seized by Andrews.

CCTV footage shows Andrews, 37, dragging Somerville across the lobby and hurling her into a cell. She smashes her head on the hard floor and blood is seen spattered across the cell.

Somerville called for help and, as she was driven to the Royal United hospital in Bath, she says blood began to spurt from her mouth and she thought she was going to die.

She needed stitches in a gash above her eye. Still pictures taken of her show her left eye is almost completely closed and her shirt is covered in blood.

"I still find it hard to watch the images of me staggering to my feet with blood pouring from a head wound because I can remember how terrified I was," she said.

"It seems utterly barbaric that an innocent person could be treated in such a horrific and violent way and then left alone.

"I could have died. What happened to me was extraordinary, terrifying, and no one should ever be treated in the same way again.

"My vision is still affected. It's as if I am looking through a cloud. And the whole of the left side of my face is now lower than the right, like a stroke victim."

Somerville said she was shocked that the custody officer must have known his actions were being captured on CCTV but still felt able to act as he did.

The incident was reported by another police officer and Andrews was removed from duties that brought him into direct contact with the public.

The matter was referred to the Independent Police Complaints Commission, which was satisfied that an investigation should be conducted by Wiltshire police. This investigation resulted in a file being sent to the Crown Prosecution Service and ultimately to the conviction of Andrews.

The police have promised to learn from what happened.

There are three custody suites in Wiltshire – at Swindon, Salisbury and Melksham. A fourth suite in Marlborough is used in exceptional circumstances. In the three suites, 73 cells operate 24 hours a day.

The police say 16,000 people a year are dealt with in these centres, meaning more than 30,000 have been handled since the attack on Somerville. In that time there have been 13 complaints of assault but the police say none of the allegations have been substantiated.

However an inspection of custody suites by Her Majesty's Inspectorate of Prisons and Her Majesty's Inspectorate of Constabulary in September 2009 – more than a year after the assault on Somerville – revealed a series of concerns.

It found that the use of force on detainees was not collated and analysed to identify trends.

The report said: "Use of force should be monitored centrally to enable managers to identify patterns and monitor trends, and to assure that it has been deployed appropriately and proportionately."

It also highlighted concerns with complaint procedures. "Systems for dealing with complaints were confused, and neither staff nor detainees were clear of the process. We were not convinced that the force had communicated either to detainees or to staff how complaints in custody should be dealt with."

Most of the cells surveyed failed to pass safety tests because of the presence of ligature points. Some of these were addressed during the week of the inspection.

The report went on to say that the "specific needs" of women and people with disabilities were "not well met". Showers were rarely offered, some toilets lacked privacy and toilet paper was not routinely supplied.

However, the inspectors said they observed "good relationships" between staff and detainees and "in most respects, police detention is well managed".

The Wiltshire police force has formally apologised to Somerville but said it was not possible to provide 100% assurance that its guidelines would not be broken again.

Assistant chief constable Patrick Geenty said: "Whilst that is unpalatable, the reality is that policing is complex and difficult and again we repeat, there can never be any excuse for excessive behaviour by police officers or police staff. We are determined to learn any lessons that emerge from this case and we will not shirk from our responsibility to continue to reduce the chance of any recurrence of such incidents.

"There is no greater responsibility on the police than the care of people in our custody and considerable effort and great importance is placed on ensuring that our processes, systems, training and behaviours are directed towards facing up to that responsibility at all times."