So why did they make that argument?
A lawyer who has worked on this case, has expressed his views on this an I have quoted him below -
"I need to be a bit careful what I say about this because I have discussed it with Jeremy Bamber so I know his own views on the matter. Obviously he was at the trial and I was not so I have to take seriously Jeremy's views on how his case was handled. I do not want to breach confidences, but I do not think Jeremy would mind me saying that he felt that given the constraints imposed by the non disclosure of evidence his two barristers did a reasonable job.
With the benefit of hindsight obviously there are aspects of the case which could have been handled in a different way. However, I suspect that the defence view was that this was a weak case based almost entirely on circumstantial evidence, and that raising doubts about the prosecution arguments was likely to be a better course tactically than, for example, launching a direct attack on relatives alleging a conspiracy against Jeremy.
As far as the evidence of the financial gain to family members was concerned as I understand it everyone was misled, including the prosecution barristers and the judge. It would have been difficult for the defence on its own to get to the bottom of what was a fairly complex situation.
My own personal view is that although it appears that Geoffrey Rivlin and his junior did a competent job at trial more might have been done in advance of the trial by the solicitors acting for Jeremy in pursuing some lines of enquiry. I may be unfair in saying this, but I suspect that there are other firms of solicitors who might have been more effective."
http://jeremybamberforum.co.uk/index.php/topic,1122.msg34540.html#msg34540
"A reasonable ground can be an inference which may be drawn from the surrounding circumstances. In this case just dealing with the silencer evidence: i) JB denied being responsible for the murders therefore the defence case was that Sheila had to be responsible, ii) the rifle was discovered by police without the silencer fitted, iii) the silencer was subsequently said to have been found by a relative days later in a box in the cupboard under the stairs, iii) the silencer was removed from WHF, examined and handled by several of the relatives and retained by them for several days, iv) the FSS found blood inside the silencer which was either Sheila's or, less likely, a mixture of Nevill's and June's, v) items of Sheila' bloodstained underwear were removed by a relative from WHF, vi) although possible, the suggestion that Shela had used the rifle with the silencer fitted initially then removed it, placed it in the box in the cupboard and then shot herself, was an unlikely scenario, vi) the only other explanation for the presence of the blood inside the silencer was contamination, either accidental or deliberate.
Against that background Rivlin would have been perfectly entitled to raise the suggestion of contamination, even deliberate contamination, because that was an inference which might be drawn from the evidence. I would go further and say that if Jeremy had insisted Rivlin would have been under an obligation to put such allegations to witnesses and he would have been in breach of the Code of Conduct if he had refused to do so."
http://jeremybamberforum.co.uk/index.php/topic,4121.msg171157.html#msg171157