So based on recent discussions, how's this for a hypothesis. Pure conjecture, but it uses a lot of known facts, plausible sightings, and DV hypotheses. I've put these in bold. Italics are my own speculations.
CV is in cahoots with someone who has a beef with SJL. Let's call him A N Other (but it could be a they - the couple DV won't discuss are the obvious possibility here). Perhaps she's pulling out of the business venture they were planning. Maybe this is going to bring to a head his money troubles. Maybe he tried it on with her, and he's afraid his wife will find out and realise why she's gone cold on the business idea.
He wants to confront her, but she is avoiding and won't meet him. He needs to trick her into a meeting in a discreet place. So he lifts her stuff or someone or perhaps his mate CV does, while she's making a phone call to AL from the pub payphone, and he has CV "find" it.
Next day, after she speaks to CV's partner 'Karen', this is enough to lure SJL to the pub to retrieve her diary. CV goes along with this, favour to a mate perhaps. But to his horror, when SJL unsuspectingly turns up at the PoW, A N Other kills her.
CV is appalled - but A N Other warns him that 'like it or not pal, you're an accessory now, because you got her here. You get your @rse in gear and you help me fix this. Get rid of her car and then get back to the PoW ASAP to help dispose of her body under that floor'.
CV drives her Fiesta over the river and dumps it at random. He gets a cab and Tube back. He knows this is a big deal and the car will be looked for and found. The cabbie may remember him, so he plants some misinformation. He tells the cabbie he saw a couple 'having a right ruck'. This 'ruck' mislocates the place SJL came to harm from the PoW to somewhere near where he left her car.
Back at the pub, he and A N Other are hiding SJL under the pub floor when 'Karen', CV's partner, somehow gets wind of something afoot and is suspicious. A N Other leaves. A N Other's wife wants to know how the conversation with SJL went. Both men give their women some spiel.
SJL is reported missing. CV and A N Other breathe a huge sigh of relief as the police charge off in completely the wrong direction after Mr Kipper, her cover story that's a lucky, lucky break for them.
A N Other's wife had been planning to meet SJL for lunch that Monday but cancelled it. She's distraught and thinks had she not done so SJL would be alive. She helps DL establish the SLT.
'Karen' contacts the police about SJL's intended visit to the PoW, but it's CV that they meet and interview. He asks A N Other what to tell them. A N Other says give them her diary, it's a blizzard of suspects - all the men she's slept with; it'll keep them busy for months. CV obediently tells the police she was never there.
'Karen' becomes (correctly) convinced that CV knows something about SJL's disappearance, because he behaved so weirdly the day she vanished and since. She knows SJL was expected that day at the PoW. She may even have seen her there. It dawns on her what she actually saw, and appalled, some months later 'Karen' ends it with him.
A N Other's troubles materialise and his wife, putting 2 and 2 together like 'Karen', divorces him. Neither speaks about SJL ever again and not a single one of the many documentaries or articles ever, ever mentions them. She reinvents herself as a TV talking head, but despite publicity-seeking, she never milks her connection to SJL.
The PoW suffers a blowfly infestation because there is a body under the floor. There's a Crimewatch reconstruction on TV, but nobody comes forward to identify themselves as the 'James Galway' lookalike taxi fare who mentioned 'a couple having a right ruck'. This was CV, who obviously wasn't coming forward.
A year later CV's re-interviewed, and embellishes his story with made-up phone calls to suggest that someone else intended but failed to meet her there.
Four people know or suspect what happened to SJL: CV, 'Karen', A N Other, A N Other's ex-wife. Mesmerised by the Mr Kipper fiction, then by the JC story, no journalist or cold-case investigator bothers any of them for 35 years.
Until DV comes along.