This has been discussed here several times before. There is an excellent academic page on the disaster which gives a useful overview of the tragedy and subsequent Tribunal here https://www.nuffield.ox.ac.uk/politics/aberfan/home2.htm
There is also the parliamentary debate of 26th Oct 1967 which provides much interesting background to matters such as why the government refused to accept Robens resignation as well recognition that tip safety was not, prior to the disaster, a matter of concern. Underground works were the focus of OSH in mining
It is too easy to focus on Robens as the key player in this disaster and thus miss the reality. Apart from anything else, the NCB was not like a Board of directors with a Chairman and/or CEO. There was clearly a classic set up of a poorly governed and organised undertaking. The National Coal Board was a huge undertaking ‘managed’ through central command and control. Nearly 60, 000 were employed in Wales alone. It was doomed to fail at some point and it did catastrophically at Aberfan on that fateful morning.
We have to try to understand that the significance of tip safety was simply not perceived to be a major safety issue. All the engineering and control was directed at the underground activity. There was no legislation at all re tip safety.
If you check Hansard http://hansard.millbanksystems.com/commons/1967/oct/26/aberfan-disaster
you can read that the Minister of Power in the 1967 debate admitted that “One of the aspects most disturbing about this incident is the sheer lack of technical expertise which was available. I freely confess that, on the day after the disaster, when I talked to the Mines Inspectorate in my Ministry, I rapidly discovered that which I did not know before—and the hon. and gallant Member for South Fylde (Colonel Lancaster) will be aware of this—that this was not the usual mining expertise that was needed. In fact, we had to look for an expert in soil mechanics.” A clear recognition that the whole matter of civil engineering above ground had never been adequately addressed by anyone in the industry.
I mention these points not in defence of Robens or the NCB because they undoubtedly got things badly wrong after the tragedy, but to illustrate the cultural and political environment of the time.
The fact that the Tribunal, despite naming nine individual NCB employees and officials for particular criticism, went on to conclude that “The Aberfan Disaster is a terrifying tale of bungling ineptitude by many men charged with tasks for which they were totally unfitted, of failure to heed clear warnings, and of total lack of direction from above. Not villains but decent men, led astray by foolishness or by ignorance or by both in combination, are responsible for what happened at Aberfan.”.
The Sec of State for Wales during the October 1967 debate said “In a moving passage in paragraph 207, the Tribunal expressed its belief that Whether or not named or adversely referred to in this Report, there must be many today with hearts made heavy and haunted by the thought that if only they had done this, that or the other the disaster might have been averted. Of these, some will blame themselves needlessly; others, while blameworthy in some degree, will condemn themselves with excessive harshness; yet others must carry the heavy burden of knowing that their neglect played an unmistakeable part in bringing about the tragedy. There, in my view, the matter ought now to be allowed to rest.”