Robert ('Uncle Bobby') Boutflour disliked Jeremy intensely, even at the time, so why wouldn't he at least tell Ann or David, or at least his own wife? He can't say he forgot about it because, as you recognise, and as we all know from the evidence, it was a memorable incident.
I think there are two possibilities:
(i). Jeremy did say it but it was just a joke or flippancy. Robert knew this full well, but later remembered it and decided to twist Jeremy's words out-of-context because it could be put across as further incriminating of Jeremy. For his part, Jeremy may have been left with no option but to refute it, or he may even simply have forgotten the whole incident.
OR
(ii). Robert lied outright: he either invented the incident, or it was a real incident but Jeremy said something different.
I would say (i) is the more likely possibility. It sounds very much like the sort of thing Jeremy would say as a joke or in a spirit of flippancy. This would explain why Robert never mentioned it to anybody at the time. Had he thought Jeremy was at all serious, he would clearly have informed Nevill, or at least his wife or some other relative.
I'm sorry but I just do not find very credible the idea that Jeremy would say such a thing in a serious way and Robert would tell no-one but then suddenly remember it all months later. I acknowledge it is possible, but it doesn't seem very likely to me. On the other hand, Jeremy was known for having a teasing/jokey manner, and having dismissed it as silliness at the time, Robert might later have remembered the incident and decided to present in an incriminating way.
Didn't Robert Boutflour also meet with the police and pressure them to investigate Jeremy?
I also thought it was Robert Boutflour who alerted the police to the silencer. He had gone to Witham Police Station to demand a meeting with a senior officer and in passing he mentioned the silencer, as he assumed Ann and Peter had already handed it over. When the police said, 'What silencer?', it became clear the evidence had not been collected and Stan Jones then arranged to do so.
Robert Boutflour was the patriarch of the Boutflour/Eaton side of the extended family and, despite being only a minor witness at trial, I would say his moral influence on detectives was a linchpin of the whole case.
What I have just told you is, I believe, considered canon. Yes, the family overall acted in concert, but my point is that Robert Boutflour was a major driving force - perhaps the driving force - behind the conviction of Jeremy Bamber.
It was Robert Boutflour who wrote that rather disingenuous note to the jury in reply to their famous question.
He also maintained to the jury that he measured his wealth in friends, not 'LSD'. I don't accept that. On the other hand, I must say that the idea he, or any other family member, would frame Jeremy is an extraordinary claim and must meet Laplace's principle.
The Laplace principle is not insurmountable, though.