For the next two weeks, Shaw, 46, is shadowing his predecessor, Sir Peter Woodhead, before taking over his job on 11 October.
It is a task that has grown beyond expectations, with more than 2,000 complaints a year. About 30 per cent of those were eligible for investigation last year, more than a third of which were upheld.
The office was set up in 1994, in the wake of the 1990 Strangeways riots. Lord Woolf's subsequent report maintained that it was essential to have an independent adjudicator as it was impossible to reform or rehabilitate prisoners who felt they were being treated unjustly.
Prisoners can complain to the ombudsman only once they have exhausted the internal complaints process. While most prisoners make their complaints themselves, a proportion come via solicitors.
For Simon Creighton, solicitor for the Prisoners Advice Service before joining a criminal law practice: "The prisons ombudsman is invaluable because the legal remedies are so limited and the majority of complaints are not ones to litigate on.
"I use the ombudsman as a first resort before the courts in the majority of cases. For me the whole point of the office is, first, to stop people needing to go to law, and secondly, to look at the overall justice and fairness of a decision - not whether there is a legal precedent.
"My hope is that Shaw will be active in drawing out themes from the individual problems that he has to confront, and putting them to the Prison Service and the Home Office."
Certainly, Shaw's appointment has prompted jubilation from the penal lobby ("brilliant", according to Paul Cavadino of Nacro) and is a marked change from five years ago when he was rejected for the job by the then Home Secretary Michael Howard as being "too liberal". Creighton was concerned that Shaw's background might cause him difficulties in "getting the trust of some prison staff to identify him with what they see as a dangerously left-wing reform group and raising prisoners' expectations too high."
"I have got baggage," admits Shaw. "I remain of the view that private- sector management of prisons is morally objectionable, but it is equally clear that, in terms of quality of relationships between prisoners and staff, private prisons have a great deal to teach the public sector - there is no queue of prisoners wanting to transfer from the Wolds or Doncaster to Hull or Leeds."
What will help him in his new role is the promise his predecessor has won from the Home Secretary, Jack Straw, to put the office of prisons ombudsman on to a statutory footing to guarantee its independence, as soon as there is legislative time.
For Creighton, any legislation should also give the ombudsman binding powers to make the Prison Service act on his recommendations, which they can at present reject.
"Because he doesn't have binding powers, it dilutes the impact of his decisions and means that in very important cases we would choose to go to court," he explained.
However, Shaw is not convinced such a move would make him more influential. "I already have a wider remit than the courts, which only determine whether a prisoner's rights were properly enforced. My job is to determine whether prisoners have been treated with fairness and dignity, and in line with the spirit, not just the letter, of the rules.
"I have taken the job because I want to do justice to the individual prisoner's complaints. But if that were all I did, I would have failed. My role ought to be as a catalyst to help the Prison Service improve its performance across the board."
For Shaw, the next legislation to have a big impact on his work will be the Human Rights Act, which comes into force next October and will open up many aspects of prison life to legal challenge. It could affect such diverse aspects as setting prisoners' security category, allocation of an inmate to a particular prison, deciding whether mail can be read or stopped, the parole process and the legality of adjudications. One likely consequence will be a review of the 1952 Prison Act, he believes.
Grievances about adjudications, whereby a prison governor can impose up to 42 "added days" to a sentence for breaches of prison rules, already form a quarter of all the complaints received by the ombudsman and 44 per cent of those that are investigated. Sir Peter's concerns about the internal complaint system has prompted a review.
Shaw says that it is one of the "dismal" features of the current system that nearly three-quarters of prisoners who complain to the ombudsman are ineligible; they have "jumped the gun" and have failed to go through the correct procedures, having been frustrated by delays. He is hoping to encourage speedier responses from the Prison Service and more local resolutions.
He is hopeful that he will be able to achieve his aims without the need to "make waves". Sir Peter survived bruising confrontations with Howard, who curtailed his powers after finding that the former Deputy Supreme Allied Commander in the Atlantic could not be bullied. Straw has since restored the ombudsman's terms of reference.
One touchy issue Shaw will have to tackle with the Home Office is money. The office budget is pounds 750,000, which provides for "16-and-a-half" members of staff, including three assistants and seven investigators. But that will be insufficient if the work load rises as anticipated with the Human Rights Act, the record prison population and Shaw's own determination to encourage more inmates to use his office. "There is time enough to pursue that," he says.
For Shaw, the sort of grievance that excites him is that of the prisoner who is undergoing an education or therapeutic course, and is transferred at short notice to a prison where that is not available. "You could certainly argue that the prisoner has a legitimate complaint and that the Prison Service is failing to provide effective, rehabilitative programmes to reduce the number of future victims."
He is also keen to see that the ombudsman plays a wider role; for example, in the investigation of deaths in custody.
Douglas Hurd, the former Tory cabinet minister who now chairs the Prison Reform Trust, warned Shaw not to become a "bureaucrat". "I don't think that is the fear uppermost in people's minds," says Shaw.Reuse content