Anyone who thinks criminal cases are really just civil cases but with a higher standard of proof as to the ultimate issue will not be surprised by the growth in complexity of pre-trial procedures. Those relating to disclosure of information by the prosecution to the defence received some consideration by the Privy Council in McDonald v HM Advocate  UKPC 46 (16 October 2008).
The need to ensure that an accused person receives a fair trial seems to be taken to mean that, as far as is possible, the "parties" (to use an expression from the civil side of the law) should be put on an equal footing. The assumption is that the accused will have to prove his innocence, regardless of what might be said about the standard of proof.
Fraying of the golden thread has led to development of a new "golden rule", stated by Lord Bingham in R v H and C  2 AC 134 and referred to by Lord Rodger in McDonald at para 50:
"...Put shortly, the Crown must disclose any statement or other material of which it is aware and which either materially weakens the Crown case or materially strengthens the defence case...".
The application of this rule requires development as cases arise. Reference was made in McDonald to the current preparation of a legislated code concerning disclosure in Scotland.
In New Zealand we have new legislation that has not yet come into effect: the Criminal Disclosure Act 2008. This will impose obligations that are much wider than those of the golden rule. [Update: the Act commenced on 29 June 2009.]
In McDonald Lord Rodger was particularly concerned with the problems that will arise when large volumes of information come under consideration for possible disclosure. He pointed to the different functions of the prosecution and the defence in an adversarial system, 60, and observed that there is no way of avoiding reliance on officials carrying out their duties conscientiously. He considered that where the defence makes only a general request for disclosure it would need to be able to satisfy a court that the material it sought would have a legitimate bearing on an issue before disclosure would be ordered (76).
The narrow rule/wide request/narrow enforceability approach is not taken in the New Zealand legislation, which has more of a wide rule/wide request/fairly wide enforceability structure.
Recently there was some public concern over the extent to which the defence could obtain knowledge about people who were not involved in a particular case although their personal information had been gained in the course of police inquiries. That concern is heightened where accused persons represent themselves and have access to that sort of information. Perhaps that incident involved an over-eagerness to be candid with the defence, although it could also indicate an unwillingness by the police to devote time to the sifting of information to isolate that which might assist the defence. The Act is limited to "relevant" information (s 13(2)(a)), and this would not authorise the police to simply dump as much information as possible on the defence.