Separate Crime reference files were kept at Southend police station for the five deaths, than were created at Chelmsford police station - for example, at Chelmsford a general Crime reference of 'SC/688/85' was created which deal with the five deaths collectively, this was because at Southend police station, Sheila Caffell was also part of the drugs squad undercover operation, and because she ended up dead at the scene whilst the drugs squad operation was ongoing, Southend police took precedence over Chelmsford in creating individual crime reference files for each of the five victims, including Sheila who as I say was one of the suspected target criminals in the drugs investigation (I have these Southend police Crime reference numbers in my possession)! Freddie Emani was another who was being investigated as part of the same undercover operation, so was Julie Mugford' and all the other students who lived in the same building in bedsits close to her room...
All the missing evidence is being kept at Southend police station, the defence never got to see hardly any of this material, and would only have been given access to some of it if they could prove that it's disclosure would tend to prove let's say Jeremy Bambers innocence! The problem would have been then, and it is the same now, that the defence would have had to specify the evidence they are seeking to rely upon and give a valid reason for why that document or that evidence should be released, without having it confirmed beforehand that such a document or such information actually existed at all in the first instance! (Pii rules of evidence, and disclosure apply)...
The drugs operation files held at Southend police station is the way forward - they have got evidence proving that Jeremy Bamber did receive that early morning telephone call from Neville Bamber, it's contained and has been retained for over 33 years under a telecommunications act warrant (pii), which can in unusual circumstances be obtained for use in Jeremy Bambers defence providing he can specify the document / evidence he wants or needs, and gives a justifiable reason or ground why that information or documents should be released!