Steve is wrong and Roch is correct. Counsel is entitled to put any accusation to a prosecution witness, with certain exceptions. It was open to Rivlin to ask a witness whether he had tampered with the silencer. He chose not to ask the direct question for tactical reasons. I do not have time now to set out the entire position but a few years ago I posted a lengthy explanation of the legal position in response to another member. I gave an example of an Old Bailey case with which I was very familiar. A search should find the post.
ngb, Anthony Arlidge questioned the scientist and showed him a list of names, which didn't include the relatives' names and results. The scientist concurred with Arlidge that nobody else on the list shared blood groupings with Sheila Caffell; the inference being that the blood in the SM could only have come from Sheila Caffell. Therefore the jury were led to believe that the blood groupings in the SM did not match anyone else's blood groupings.
I assume the jury were aware that the relatives discovered the SM after being given the keys to the farmhouse by Jeremy Bamber, subsequent to police searches.
If Rivlin had cross examination the scientist regarding the blood groupings of Robert Boutflour as (per detailed by the scientist in his written communication) do you think this may have unsettled the jury regarding the SM evidence? Could it have also restrained Justice Drake in his summing up?