Scipio, is the excerpt you've posted from something post trial eg COLP? It states that although the items would not have been destroyed if the murders were by an unknown person "no further evidence would have been forthcoming had they been retained".
DC Hammersley's trial testimony states he removed everything of evidential value from the bed in the main bedroom.
No officer observed any blood on the bedding NB's side.
It is from the Dickinson Report. The Report noted how they destroyed most of the bedding within days and says while such was against protocol it ultimately would not have contained any physical evidence from the killer thus doesn't matter.
No officer was asked to record in documents whether they observed blood on Nevill's side. The only bedding taken into evidence was: 1) the bottom sheet of the bed, 2) June's pillows. Everything else was destroyed. They never did any testing on the sheet or pillows. Thus Dickinson felt they would not have done any testing on any of the other bedding either had they retained it and there is no reason to believe Jeremy's blood was on it so no reason to believe testing of the bedding would have helped establish his guilt.
The police felt the casings and bullet that grazed Nevill proved he was shot in the bedroom. They saw no need to try using blood evidence to establish it. Prior or at trial no one questioned that Nevill was shot in the bedroom based on a claim there was a lack of blood on the bed and floor on Nevill's side thus there was no need to ask police about the blood on his side of the bed. How much they would remember is questionable anyway given they paid such little mind to the bedding. The police never mention blood on June's side of the quilt either. They didn't mention it because they thought it was of no investigative value which is also why they destroyed it. the fact they don't mention it doesn't mean it wasn't there we can see there is blood on her side of the quilt
Subsequent to the convictions is when blood experts were asked to assess photos of the bedding and thus when blood experts paid attention to the bedding. Photos show blood on the pillow and bottom sheet of June's bed but there are no photos that were taken of the sheets on Nevill's side. The only photos of his side were with the quilt blocking view of the sheets and most of these were taken at a distance the only one close up is a photo of Sheila which happens to include a portion of the quilt and it looks like there is blood on the quilt in that photo. The blood experts didn't say anything about the blood on the quilt on June's side of the bed because it told them little. What they paid attention to was blood on the pillow that got there by her bleeding head touching the pillow- which means she was shot in the head while lying down in bed not in the process of getting out of bed- and the bottom sheet had transfer marks showing her bleeding exit wounds were touching the sheet. There were no similar pictures taken of Nevill's sheets to analyze though he suffered no exit wounds that could cause the kinds of transfer stains they were looking to assess anyway. Nor did he suffer a head wound in a location that would allow transfer of blood to his pillow even if he had been lying down when shot.