Surviving victims and relatives of those who died as a result of receiving infected blood and blood products from the NHS in the 1970s and 80s will gather in a few weeks at the Methodist Central Hall in Westminster.

After six years of taking evidence, Sir Brian Langstaff’s public inquiry will finally unveil its report there on 20 May.

There is no doubt that what occurred was a tragedy and scandal, indeed one of the very worst in this country’s history. Thousands were killed, including children, after being infected with HIV and hepatitis. Many more lives were ruined. And there has been no shortage of apologies in recent years. As the then prime minister, David Cameron, told the House of Commons in 2015: “To each and every one of these people I would like to say sorry on behalf of the government for something that should not have happened.”

But for all the apologies, there has been a distinct lack of candour about quite what the government is apologising for – and who should take the blame.

For too long, campaigners say, the suggestion has been that it was all just an unfortunate tragedy that unravelled despite the best intentions of ministers, civil servants, the pharmaceutical industry, the NHS and the health department.

What those planning to gather at the Methodist hall say they want to hear is not that those engulfed by the scandal had suffered “very bad luck”, as the former prime minister John Major had said in evidence to the inquiry, or even that some things could have been done differently.

They will gather in Westminster, the campaigners say, with the expectation of an acknowledgment that the entire affair was not only entirely avoidable but resulted from corporate greed, state negligence and a culture of cover up.

According to the inquiry, an estimated 2,900 people died as a result of having been given infected blood or blood products in the 1970s and 80s. Of those, 1,170 had suffered to varying degrees of haemophilia, an inherited condition in which the blood does not clot normally.

They had been given a blood-clotting product known as Factor VIII – but a cheap version of it. Much of the blood had been imported from US prisons, and taken from high-risk donors such as sex workers, drug addicts and alcoholics who were paid. That decision was not inevitable. There were other options.

As reported on Sunday, one of the suppliers, Immuno IG, recognised that there was a “significantly higher hepatitis risk” from a Factor VIII product known as Kryobulin 2, made from US plasma compared to its original product, Kryobulin 1, sourced from unpaid donors in Austria and Germany.

The pharmaceutical company had concluded that “the British market will accept a higher risk of hepatitis for a lower-priced product”.

In 1975, the then health secretary, David Owen, promised Britain would become self-sufficient in blood products to ensure vulnerable patients were put at minimal risk. But a different path was taken. Owen moved on to be foreign secretary, and in 1979 the Conservatives took power and the investment was not made.

The government continued into the mid-80s to buy blood from prisons in Arkansas, despite the fact that the US Food and Drug Administration had banned its sale in the US in 1983.

When the horrors of HIV/Aids emerged in the 1980s, further decisions were made. Dr NS Galbraith, the founding director of the Communicable Disease Surveillance Centre in England and Wales, wrote to the Department of Health and Social Security in 1983, saying: “All blood products made from blood donated in the US after 1978 should be withdrawn from use until the risk of Aids transmission by these products has been clarified.” But they were not withdrawn. The secretary of state Ken Clarke, told the House of Commons: “There is no conclusive evidence that Aids is transmitted by blood products.”

Further decisions were made to destroy government documents, conceal HIV test results from those who had been infected and to wait until 2017, and only then under pressure from a group litigation, to launch a public inquiry.

Langstaff now has a decision to make – and campaigners are hopeful he will make the right one. Last year, when summarising the conclusions of an interim report about compensation, he wrote: “My conclusion is that wrongs were done at individual, collective and systemic levels. Not only do the infections themselves and their consequences merit compensation, but so too do the wrongs done by the way in which authority responded to what happened.”

It is also perhaps reassuring that the inquiry’s final report has been repeatedly delayed by the “sheer volume” of letters of warning that Longstaff is obliged to write to those who will be criticised.