The common law recognised the problem and, more recently, Parliament has enacted the Serious Organised Crime and Police Act 2005 Chapter 2 (Offenders assisting investigations and prosecutions). The Court of Appeal (Criminal Division) has held that the Act did not abrogate the common law power - R v P and Derek Blackburn  EWCA Crim 2290. The Act came into force on 1st April 2006.
Prior to the coming into force of SOCPA, where a person convicted of a crime had offered assistance to the law enforcement authorities, the courts had taken that assistance into account when sentencing. The key principles are set out by Lord Bingham CJ in R v A and B  EWCA Crim 3529. (Link unavailable).
Since the enactment of SOCPA,
the question has come before the Court of Appeal on a number of occasions: most recently R v ZTR  EWCA Crim 1427 (Lord Thomas CJ, Saunders and Edis JJ).
ZTR had been convicted of murder. After SOCPA came into force, ZTR was asked to provide assistance to the Police in matters arising after his conviction and sentence for murder. Did he now qualify for a reduction in the minimum term he must serve? His case did not come within the SOCPA scheme and so the Court of Appeal was asked to extend the common law.
The Lord Chief Justice sets out the essential common law principles at paragraphs 8 to 11. Where a defendant denied guilt but was convicted and sentenced following a contested trial without supplying valuable information before sentence or expressing willingness to do so, the Court of Appeal (Criminal Division) would not ordinarily reduce a sentence to take account of information supplied to the authorities after sentence.
The applicant argued that the principle of reducing sentences for assistance provided ... operated on pragmatic considerations that a reduction should be offered whenever it might encourage a person to provide the necessary assistance to the authorities. It could therefore be offered at any time assistance was offered, whether before or after sentence. The common law could therefore adopt, on the basis of pragmatism and utilitarianism, the principle in the 2005 Act which permitted a sentence to be reduced for assistance given subsequent to the time of the sentence.
The Court of Appeal rejected this invitation. At para 19 the Lord Chief Justice said:
"Although we see the force of the argument, good reasons would be needed to change so clearly established a principle. We have carefully considered the arguments based on utilitarianism and pragmatism; these have, as is evident from the judgments of Lord Bingham and Lord Judge, always been present. We can see no good reason to depart from the established principles. There are also two countervailing considerations. First this court would not be acting as a court of review, but rewarding someone for good behaviour during his sentence. That is not this court's function. Second, experience has shown that some may be motivated to manufacture assistance after conviction in the hope of a reduction in a long sentence. Nothing should be done which might encourage this."
The court went on - at para 21 - to make an interesting suggestion:
"As we have concluded that this court should not, in the circumstances of cases such as this, exercise any jurisdiction it might have at common law and as the Parole Board has no power to reduce the minimum term, it must remain open to an applicant such as this to consider applying to the Home Secretary to exercise powers under s.30 of the Criminal Justice Act 1997. The power of the Secretary of State under s.30 of that Act was considered in R v McLoughlin  EWCA Crim 188;  2 Cr App R (S) 40 at paragraphs 31-36. We express no view on whether circumstances arise on the facts of this case which would entitle the Secretary of State to exercise his power under s.30." [Sic].
The reference to Criminal Justice Act 1997 appears to be in error and it must mean the Crime (Sentences) Act 1997 section 30 which, as enacted, enabled the Home Secretary to release a prisoner on licence if there were compassionate grounds to do so. In R v McLoughlin, the Court of Appeal effectively extended this power to embrace release, in some circumstances, of those serving whole life sentences for murder. Whether the Court of Appeal's latest suggestion is taken up (and it might well be) remains to be seen. Only then will we perhaps find out whether section 30 is being pressed too far. Law and Lawyers discussed McLoughlin in a post of 18th February 2014.
R v A  EWCA Crim 1803
R v P and Blackburn  EWCA Crim 2290- this statement of common law pragmatism in this area is at para 22:
"There never has been, and never will be, much enthusiasm about a process by which criminals receive lower sentences than they otherwise deserve because they have informed on or given evidence against those who participated in the same or linked crimes, or in relation to crimes in which they had no personal involvement, but about which they have provided useful information to the investigating authorities. However, like the process which provides for a reduced sentence following a guilty plea, this is a longstanding and entirely pragmatic convention. The stark reality is that without it major criminals who should be convicted and sentenced for offences of the utmost seriousness might, and in many cases, certainly would escape justice."
also, for section 30,
R v McLoughlin and Newell  EWCA Crim 188