So here is some pure speculation, but it fits the inferred facts.
SJL was, according to the police, "a modern young woman". Her older diaries were provided to the police. Let's suppose, just for argument's sake, that "a modern young woman" is a police euphemism for "SJL had slept with 250 to 300 men in the last five years, and 20 so far in 1986, of whom AG was just one of several she had on the go at that time".
There is a lot more of this about than one might suppose. When I was 21 I was friends with a very pretty 19-year old who had slept with 20 men already. When I lost touch with her when she was 35 she was Virgin cabin crew and "up to so many I can't remember but at least 500 by now". She did it because she could.
Look again at SJL:
https://crimegirlgang.com/wp-content/uploads/2020/01/Suzy-Lamplugh-thetimes.jpg
She could have, too. She could have had anybody she wanted. And maybe she did.
When AG went away on holiday, she had several other strings to her bow who kept her happy in his absence. He did not, and when he gets home, presumably very keen indeed to resume relations with her, is startled when she instead arranges her weekend without him and then dumps him. He was never really her "boyfriend" boyfriend. He was someone she knew and slept with, but so were others, and she had lost interest. But she agreed they could stay friends as they were of the same clique.
On Sunday, SJL met one of her "others" at the pub. She mislaid her diary. She could not have anyone reading it because it would blow the gaff. It listed everyone she had slept with and intended to sleep with that year. She needed it back and she could not ask anyone to pick it up - eg her lodger - because what if he's nosy and reads it? What if it says "I would actually do my lodger. I do wish he'd get it together and ask"?
So she needs it back. So she has to go. What happened next? No idea.
But what
do we know? We know AG and SJL had split up, but her parents didn't know. He was a nice, good-looking bloke, in a profession, with prospects. They'd have been
very happy for her to have become Susanna Leegood.
He appears with them at press conferences, even though the boyfriend is usually a suspect. This happens because the boyfriend is a suspect, i.e. it's one of the
other 20 boyfriends who's a suspect, not AG, who's got an alibi.
The 1988 book hinted at the promiscuity, and it offended her Wesleyan Methodist parents so much it was never elaborated on. It was probably right to suppress it. First, there's nothing wrong with it, and second, the story then changes from "attractive middle-class estate agent abducted" to "slapper done in by one of her HUNDREDS of blokes". At the point the victim-blaming kicks in, all public sympathy and interest are lost.
So nothing is said. But AG now knows she's slept with maybe dozens of men while she's been with him, and here he is, colluding in misrepresenting her as some sort of saint. Probably, for AG, the perp is one of that year's 20 or 30 men who she slept with and dumped, and who has now taken offence and retaliated violently. If you sleep with 250 men, the chances that one of them's an entitled psycho must be quite high. So go and interview them and ask them what they think happened. One of them knows because one of them made it happen.
Thus when DV interviews him 35 years later, he denies ever having been in that pub with her. She didn't lose her stuff there with
him. She lost it with Mr bloody Lover Boy. Hence his snapped "You'll
never find out what happened", because
he can't figure out which of them it might have been and he was actually there.
The above is pure conjecture - certainly the numbers I've used - but has been hinted at these last 35 years here and there. She was exceptionally pretty and quite able to live the pretty racy love life that her diaries apparently recount. JC or anyone like him would certainly take murderous umbrage at being dumped by someone like SJL.
Hence she is intercepted by someone she knows, who isn't prepared to be dumped.
Conjecture but fits the facts, known and rumoured....