Bamber would not want a silencer at the crime scene. For several reasons.
If he became a suspect, the police would say a silencer would be used in a silent massacre attempt.
But surely Sheila could have used the silencer and then taken it off to shoot herself, leaving the silencer by her body. That, to me, seems the more likely method for staging her suicide.
Taking a step back, we know that the rifle did not fit in the gun cupboard with the silencer on. It follows that anybody returning the rifle to the house would unscrew the silencer, but we also know that Jeremy says he went out to shoot rabbits. One argument is that Jeremy would keep the silencer on in that situation in order to be able to kill as many rabbits as possible. The other argument is that he would take it off so that the audible crack of the rifle would scare them away. It's also the case that the silencer could plausibly be off the rifle because, if it had to be unscrewed to fit it in the gun cupboard, somebody could absent-mindedly leave the silencer behind. So we're left with uncertainty.
I would normally go with the prosecution argument because the 'silent massacre' theory makes sense, and Jeremy could have miscalculated the staging and then panicked and returned the silencer to the gun cupboard, thinking it would then be out of the way but at the same time could be accounted-for, thus averting suspicion. It would have been simpler and better to leave the silencer by her body, but the simpler solutions aren't always apparent in a situation like that.
The difficulty is that to return the silencer to the gun cupboard, Jeremy needed to cross the back corridor and the floor of the den, and there is no blood there. There is also no blood on the door of the gun cupboard or inside the gun cupboard (other than in and on the silencer itself). How can that be?
The police also checked the gun cupboard and did not find the silencer in there.
Jeremy also allowed the relatives free access to the farmhouse, and must have known they would seize this evidence.
It doesn't add up. The evidence points away from the gun cupboard, not to it. It's at least a basis for reasonable doubt, in my view.