The way I look at it is this: if any important aspect of the case against Jeremy relies on supposing that he was bluffing, or that he was 'cocky and arrogant', then it can't be a very good case. The burden is on the Crown at all times. We can't incriminate Jeremy by supposing he was bluffing or putting everything down to arrogance. We either have evidence for the point being advanced or we don't.
"That is what you have got to establish" - is made to seem like an arrogant statement, assuming Jeremy even used those words.
I think it is prosecution propaganda.
None of us here was at the trial. It represents one interpretation of what Jeremy said. I am well aware of the value of impressions and so on. For Jeremy's part, he turned up at court every day smartly dressed and spoke well. He even became emotional while giving evidence (something his enemies and opponents never mention). Jeremy's counsel claimed to an author that he wasn't taking the case seriously, but that's at odds with other impressions given of him, including his dress and other things. It occurs to me that his barristers and legal team may have a vested interest in speaking of Jeremy in this way so as to deflect from their own mediocre performance. Likewise, police officers and others who have a stake in Jeremy's conviction may gleefully recall how he said this or that, but they have a stake in portraying Jeremy as badly as possible and giving a slant to things that puts them in a good light and implies they are people of sound judgement.
In any event, since I was not there, I am not inclined to rely on the impressions of people who have a stake in monsterising Jeremy Bamber. I would prefer to make up my own mind. If Jeremy did say, 'That is for you to establish', some jurors may have taken it as positive and concluded that it showed great poise and confidence under pressure. Since I don't know what he said immediately before and immediately after, or I don't know what question he was answering, I cannot reach any firm conclusion.
It is anyway a true statement: it is for the Crown to establish their case.