Anne and Graeme Dixon have spent the past 13 years investigating the death of their 11-month-old daughter, Elizabeth, and the treatment she received after her birth. Now, due to recent changes in the way the parliamentary and health service ombudsman (PHSO) works, they believe they could be a step closer to finding some answers.

Elizabeth was born eight weeks early, in 2000, in Frimley Park hospital in Surrey. Immediately after birth, her blood pressure began to rise but was left untreated until she was transferred to Great Ormond Street hospital some two weeks later. By that time, Elizabeth had suffered severe brain damage. It was another 10 months before the Dixons were able to take their daughter home. Then, just days before her first birthday, Elizabeth died during the night, after her breathing tube became blocked when an agency nurse failed to maintain it.

In 2005, the nurse was struck off the nursing register and an inquest into Elizabeth’s death was held in 2009. But the Dixons, from Fleet in Hampshire, believed many questions remained unanswered and continued their own investigations. Last year, after new evidence was uncovered by the couple, Frimley Park hospital eventually commissioned an independent review into Elizabeth’s care in the days following her birth. The report found that hospital staff failed to monitor or treat Elizabeth’s high blood pressure and concluded it was overwhelmingly likely that Elizabeth’s brain damage was caused by uncontrolled severe hypertension.

“It was a bitter irony that we only discovered the true cause of our daughter’s disability while investigating her death ourselves,” Anne Dixon says. “It had taken years of sheer determination to get our evidence together and all that time, in the back of our minds, we were wondering if other babies were at risk.”

The hospital later apologised for Elizabeth’s poor care and the family approached the Care Quality Commission (CQC) with the findings of the report. In the summer of this year, after many discussions with the Dixons, the CQC pledged to work in partnership with NHS England to establish an independent investigation panel to look into Elizabeth’s case. But in August, NHS England backed out of the investigation, meaning the CQC would only be able to carry out a “thematic review” to look at the issues raised by Elizabeth’s death, as they can not look into historic, individual cases of poor care.

The case has highlighted how difficult it can be for families with historic complaints to have their concerns investigated. The CQC chief executive, David Behan, has expressed concern over what he describes as a “gap in the system” when it comes to such cases. Speaking at a CQC board meeting in September, Behan said: “There are issues surrounding how old a case is. Five years ago, I think there would have been a different response to saying a complaint is time expired, but since cases like Hillsborough, Rochdale and Rotherham, I don’t think you can say that. That argument has now gone in public services.”

In the last few weeks, the Dixons have been in contact with the PHSO about the possibility of it investigating Elizabeth’s case. The PHSO, which was set up to investigate complaints about health service providers and government departments, can provide a final, independent and impartial adjudication.

Until recently, it was rare that it took on historic cases. But as part of major changes to the way it works, older complaints could now be reviewed more regularly. By lowering the threshold for investigating cases from only investigating if the evidence showed it was likely to uphold the complaint, to now doing so if there is a case to answer, and by shortening assessment times, hundreds more cases are being considered. Between April 2013 and March 2014 figures show the total number of investigations soared to 2,199, compared to just 384 the previous year.

Although the PHSO says it won’t comment on specific cases, the ombudsman, Julie Mellor, says the changes include a shift to using their discretion more positively to help more people. “For serious cases that come to us outside the normal 12-month period specified in law, we will now positively consider whether an effective investigation is possible given the passage of time. Where we judge it possible, we will generally investigate.”

The Dixons hope the changes could finally bring them answers about their daughter’s case. “Many complaints only become historic because families have battled for years to find evidence to put forward. As a consequence, opportunities to improve safety of care in the NHS are wasted,” Graeme Dixon says. “Our hope now, is that the possible involvement of the PHSO in our case, together with the thematic review by the CQC will raise awareness of the issues surrounding infant hypertension and tracheostomy care in the community, and ultimately save the lives of other children.