Just like Operation Grange, the original investigation worked on more than one possibility. Just like Operation Grange one was that the child was alive, another that she was dead. They conducted a huge search of the surrounding area, did hundreds of interviews and followed up hundreds of sightings. The evidence that they did all that is in the files.
They didn't properly examine those closest to the child thoroughly enough and we have no evidence that Operation Grange have done that either.
If there was evidence in existence which implicates the parents- then SY would have found it during their investigations - which involved meticulous examination of all of the files - and all the other info
which we don't have. So there are two possible scenarios IMO
1. SY have found no evidence to implicate the parents - because none exists.
or
2. SY have found evidence to implicate the parents - but on instruction - have ignored/hidden/destroyed it.
Anyone who believes No. 2 needs to come up with credible reasons why an ordinary normal family from Leicester living an ordinary normal life could possibly merit a cover-up of such gigantic proportions - which if exposed would result in the ignominious ruination of the careers and lives of those who sanctioned it.
Too daft for words IMO.