Where, for illustrative purposes only, a Home Office circular regulates e.g. the use of listening devices and aural and visual procedures, the standards set by that document may be the same as those under a Stature (say RIPA or, previously, IOCA) but that Home Office circular does not mean by following its guidance it makes any acts or omissions compliant with the statutory provisions; conduct arising from following the circulars regulation, and not the statute, could be "wholly extra-statutory" and would probably contravene the European Convention on Human Rights - see Malone v Metropolitan Police Commissioner  Ch 344; cf Malone v United Kingdom (1984) 7 EHRR 14.
The above represents past history events and matters have or should have moved on since then. When RIPA was introduced it was made clear that "no" extra-statutory conduct or operations were possible arising out of that new legislation. That any acts outside of that may amount to contravention and be unlawful.
Having illustrated a simplistic model about "extra-statutory" activity by public authority and public bodies or their personnel to avoid giving advice, direction or guidance which would/could probably mean such acts may operate in parallel to the statutory provisions instead of being enshrined within them, the same principle of extra-statutory can apply across many other areas covered by other statutes, too.
Advice, direction and guidance given to facilitate the transmission of sensitive and/or unlawfuly material over public systems to aid extraction and harvesting of data from device/s might probably be "wholly extra-statutory" conduct or operations. That is even where it is a one-off case. Where advice is given to do acts which appear to go against previously stated authority in dealing with certain types of materials, the expert/examiner should record all dealings with those acts that have been instructed.
I picked up on this whilst reading books and papers dealing with judicial review of administrative action, Blackstones Criminal Practice, Archbold, telecommunications law and practice and the laws of the internet etc etc. I also noted that ACPO Guidelines and other provisions in public sector procurement documents appear not to cover any examiner/expert who enters into extra-statutory acts.
These are only my observations based upon what I read, which may assist other examiners/experts. I am not giving legal advice and I do hold out to be a lawyer. It could be from what I have read that my observations may be wrong and therefore it is always recommended to seek legal advice about instructions given or past instructions acted upon, under the belief those instructing were authorised to give such directions to do such acts in the first place.
International Conference for Communications Interception 2010