seems what happened is rather than release on parole he reached the 66,67% automatic release point of his sentence (normal for all more serious crimes other than murder with minimum tarriff) and there seems to be no way of stopping the prisoner being early released without a further Court case?
This will cause a risk will all serious/violent offenders, and might also account for cases I've recently being monitored where a middle aged man commits a further violent crime or murder having previous served a substantial sentence in his youth (which also shows there is near 0 real rehabilitation in the prison estate)