Here is the relevant part of the law that was to take effect
5. Retention of material
(a) Duty to retain material
5.1 The investigator must retain material obtained in a criminal investigation which may be relevant to the investigation. Material may be photographed, video-recorded, captured digitally or otherwise retained in the form of a copy rather than the original at any time, if the original is perishable; the original was supplied to the investigator rather than generated by him and is to be returned to its owner; or the retention of a copy rather than the original is reasonable in all the circumstances.
5.2 Where material has been seized in the exercise of the powers of seizure conferred by the Police and Criminal Evidence Act 1984, the duty to retain it under this code is subject to the provisions on the retention of seized material in section 22 of that Act.
5.3 If the officer in charge of an investigation becomes aware as a result of developments in the case that material previously examined but not retained (because it was not thought to be relevant) may now be relevant to the investigation, he should, wherever practicable, take steps to obtain it or ensure that it is retained for further inspection or for production in court if required.
5.4 The duty to retain material includes in particular the duty to retain material falling into the following categories, where it may be relevant to the investigation:
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crime reports (including crime report forms, relevant parts of incident report books or police officer's notebooks);
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custody records;
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records which are derived from tapes of telephone messages (for example, 999 calls) containing descriptions of an alleged offence or offender;
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final versions of witness statements (and draft versions where their content differs from the final version), including any exhibits mentioned (unless these have been returned to their owner on the understanding that they will be produced in court if required);
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interview records (written records, or audio or video tapes, of interviews with actual or potential witnesses or suspects);
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communications between the police and experts such as forensic scientists, reports of work carried out by experts, and schedules of scientific material prepared by the expert for the investigator, for the purposes of criminal proceedings;
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records of the first description of a suspect by each potential witness who purports to identify or describe the suspect, whether or not the description differs from that of subsequent descriptions by that or other witnesses;
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any material casting doubt on the reliability of a witness.
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Criminal Procedure and Investigations Act 1996 (s. 23(1)) Revised Code of Practice
5.5 The duty to retain material, where it may be relevant to the investigation, also includes in particular the duty to retain material which may satisfy the test for prosecution disclosure in the Act, such as:
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information provided by an accused person which indicates an explanation for the offence with which he has been charged;
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any material casting doubt on the reliability of a confession;
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any material casting doubt on the reliability of a prosecution witness.
