IIRC, something new emerged (from the inquest?) that triggered police to request more time and the displeased Coroner said to go ahead but be quick.
As I've said here before, even if police were to uncover human remains that could be identified as William's, they would still have to make a solid case that a particular individual was directly responsible for his death and that it was not accidental.
Getting someone charged and before a court would need the DPP to be convinced that there is a good likelihood of a jury convicting that person, or those persons, beyond reasonable doubt, which is a big ask in this situation IMO.
Frankly, I think the police would need a confession.