Operation Grange, according to A C Rowley, was treated as a cold case. UK police tend to revisit cold cases where there is forensic evidence because advances in forensic science, or new additions to the National DNA Database can provide an answer.
In America they have tended to focus more on new witnesses or informants. Jim Gamble at one point mentioned changing loyalties and that is what the American Police hoped to take advantage of.
http://criminology.research.southwales.ac.uk/features/cold-case-reviews/The McCann case was short on forensic evidence, so OG probably followed the American model. Hence the appeals, which did bring some new 'nuggets' of information, according to Rowley.
I wonder if they ever resolved the 195 questions they found early on? I can see unanswered questions in the files, but the people they interviewed wouldn't have answered my questions.
In the end they seem to have followed three theories, all abduction related; a planned abduction, a burglary gone wrong and an individual sex offender.
The problem I have is that while an abduction is possible, it's not the only possibility.