I believe what were called the Attorney-General's rules (officially known as the Attorney-General's Guidelines on Disclosure) was the main guiding document on disclosure back in 1984/85. This supplemented certain common law rights for the defendant on disclosure that had begun with a 1946 civil case, Bryant v Dickinson and developed over the years.
In simple terms, under the Attorney-General's Guidelines and common law, the position in 1984/85 was that criminal disclosure decisions were left entirely with the prosecutor.
The prosecutor could disclose 'unused material' (which could include draft witness statements) if the material had (quote) "some bearing on the offence(s) charged and the surrounding circumstances of the
case" (from 1981 Attorney-General's Guidelines on Disclosure).
Under case law, the prosecutor also had to consider the defendant's right to a fair trial.
I should think that even after the changes with the raft of case law in the mid-1990s, disclosure of draft statements would have been pretty uncommon and only arises in the Bamber case because of its unusualness.
My recollection is that witness statements were never actually presented as signed. I certainly never saw a signed one. In practical effect, they were escrow documents that witnesses who gave evidence then swore to in court, subject to any variations and departures in oral evidence.
I would say that it is a matter of opinion whether everything was done fairly at the 1986 trial.
The disclosure rules and guidelines allowed scope for a prosecutor to decide to make disclosures of any 'unused material' that had a bearing on the offences and/or where it would promote or assure a fair trial.
It all depends on:
(i). What the unused material is. Is it prior undisclosed witness statements or draft statements?
(ii). What the statements and other materials contain.
(iii). Why there is a difference between the draft version and the final version delivered up to the court.
(iv). Whether, in view of (i), (ii) and (iii), it was proper and/or fair for the prosecutor to withhold 'unused materials' if they might have assisted Jeremy.
I accept that, realistically, the only way Anthony Arlidge could be in any trouble is if it could be shown he had actually done something dishonest. Otherwise, regardless of my points above, surely he can just rest on the argument that draft statements were of no bearing and, as a practical matter, he won't be gainsaid by any court - to that limited extent, you are right, but it is a practical view, rather than a strict view of law or ethics.
The whole thing can be considered academic, anyway, because the disclosure rules have been superseded and were put on a statutory footing in circa 2007, and are now quite different.