The bottom line here, however the Jeremy supporters care to dress it up, is that there was a remote possibility at the time of the trial that the blood found in the silencer could have been a mix of June and Nevill's blood, which would mean that the Defence could argue that Sheila returned the silencer to the gun cupboard before proceeding upstairs to shoot herself.
We have since learned from the DNA sample given by Sheila's birth mother, Christine Jay, that the blood in the silencer bore a remarkable similarity (David1819 will correct me on the technical terms-markers I believe) to hers and therefore this theory can be all but discounted, whether it was ever a serious contention in the first place.