@ kaspar
FYI, the three most informative books to read about this case are The Suzy Lamplugh Story, by Andrew Stephen (herein "AS"); Prime Suspect, by Christopher Berry-Dee ("CBD"); and Finding Suzy, by David Videcette ("DV"). That's also actually quite a good order in which to read them.
AS sets out the facts as known before Cannan's name came into the picture, including much information about SJL's family and romantic background; CBD sets out Cannan's criminal history; DV goes back to basics and questions how Cannan was ever associated with the case, given there is no evidence he ever met SJL or was in Fulham that day - this last being the CPS' reason for refusing to charge him in 2000/1.
There is no good TV documentary on this case, as all of them start with the supposition that SJL was murdered by Cannan, and consider no other possibility. Probably a condition of police assistance making the documentary is that they don't challenge the police's preferred version; this compromises all of them.
This case feels like it should have been soluble in 1986. Nobody checked whether any sexual offenders had recently been released from the three nearby male prisons - Wormwood, Brixton, and Wandsworth. Had they done so, a list of possibles could have been assembled, and those on it required to account for themselves. This would have exposed Cannan if this was him.
A man was reported with SJL in a dark-coloured LHD BMW. There weren't that many of these in the country, so tracing them was a manageable task that might well have led to Cannan.
If SJL was abducted, then she was taken somewhere nearby. No effective effort was made to find out where else her car or any BMW might have gone; an inconvenient sighting of SJL by someone who knew her was handwaved away as probably the wrong day.
Inconsistencies in the various witness statements weren't properly bottomed out, with the result that at the end of the investigation, all the police knew of her movements was that she left the office at about 12.40.
It still seems extraordinary that someone can just disappear off the street without trace.