I do not think that EP did a proper job of gathering scientific evidence to prove that Sheila killed everyone and that she then took her own life in the bedroom, because they were absolutely certain that she was responsible for all the deaths. I am not saying they did not gather any evidence, only that there was not such an intense gathering of it, say if the incident had been treated as a full scale murder investigation from the outset...
Not to be overlooked, is the fact that scenes of crime officers gathered evidence (under SC/688/85) for use at the Coroners court, not with a view to there being a criminal trial - which was eventually pursued after the creation of the second file (SC/786/85)...
I think the investigation overall was fundamentally flawed, because in the first instance, SC/688/85 was not carried out adequately with a view to proving that Sheila did it, and how she did it, they just knew she had done it, and just went through the motions, whereas the second part of the investigation (SC/786/85) was / is based upon an editing of the first investigations material, where any suggestion that Sheila could have been involved or responsible, was edited out...