Based on the evidence, it can be safely deduced that the blood was planted maliciously by someone who has both access to Sheila's blood and a vested interest in seeing Jeremy disinherited. Who that person is I do not know for sure, but it can be narrowed down to a list of five people that meet the two prerequisites I just mentioned.
It seems David is not going to say who the 5 people were who simultaneously wanted Bamber disinherited & had access to Sheila's blood. I don't know why David, Nugs, Roch & Lookout say things they can't support.
EP had access to Sheila's blood but had no reason to want Bamber disinherited.
The relatives were rich but apparently wanted Bamber disinherited. However had no access to Sheila's blood or a vast amount of other information relating to the silencer.
So either the police fabricated the silencer & amazingly asked the relatives to pretend they found it. Or the relatives asked the police to fabricate the silencer & offerred to pretend they found it for them.