Reading through the oral evidence Acheson gave to the House of Commons Justice Select Committee in July 2016, the warnings are stark. He makes clear the National Offender Management Service (NOMS) is completely incapable of dealing with Islamist extremism. data.parliament.uk/writtenevidenc…
Source on the committee tells me: “the government was warned.”
“We found that the National Offender Management Service, the organisation tasked with dealing operationally with the phenomenon of Islamist extremism, lacked a coherent strategy to deal with the emerging threat.“ 
“There was far too much complacency, we observed, at senior level, and there was very poor understanding of risk and inadequate management of intelligence relating to Islamist extremism.”
“We found that prison staff lacked the confidence and training to confront and deter Islamist extremist ideology on the landings, and they were often fearful of being accused of being racist if they did so.“
Says the selection and training of prison imams was poor, opaque and haphazard and from an insufficiently wide set of denominations.
“In prison chaplaincies,we found numerous examples of religious literature that was extremist in nature and that sometimes contained sectarian, homophobic and incendiary information. It was freely available to vulnerable prisoners in many prisons, with no obvious control over it”
Exchange with @RupaHuq about link between understaffing in the probation service and ability to deal with extremism: “managing highly dangerous, ideologically motivated offenders is and will always be a labour-intensive business.”
At the time of the review in 2015/16 Acheson found the Prevent training given to probation staff only mentioned Al-Qaeda in relation to Islamist extremism. No mention of ISIS/Daesh. “It gave a sense of how far behind the curve those responsible for creating the training were.”
And that’s those who received Prevent training. Many probation staff did not.
“There was no strategy to deal with extremism worth the name frankly...the extremism unit in NOMS HQ was much more of a briefing unit; there were virtually no operational staff in it.”
To those who say answer is just locking up terrorists in prison for longer, evidence makes clear risk with that (unless there is total separation which isn’t always practical) is that other prisoners become radicalised by them.
Some prison imams had never even heard of Prevent and received no training: “Some of them did not even know about Prevent. None had been given any specific training about what they were supposed to do or how they were supposed to detect prisoners who were being radicalised.”
If you read through the report summary and this oral evidence you get a sense of how urgent and important the recommendations were. As Acheson said,Gove gave him his backing and intention was to implement recommendations. But then he was moved and the impetus seems to fall away..
Source on committee tells me they believe officials were against the changes proposed and ruled by them (you get the sense of that in Acheson’s evidence too). Without Gove driving it, the 68 recommendations accepted fell to 12. And none of those about probation.
The questions around this issue are not just about sentencing. It’s also about what happens to extremists when they’re in prison. Acheson paints a bleak bleak picture where the govt govt was given ample warning. So, ministers must answer, why was this allowed to drift?
