Well that's my point it's very incriminating (potentially) so why did they not mention this at trial?
There is something we don't know about the AM which helps Bamber?
Oh pleeeze...there is no smoking gun by way of a tape, video, documents or any other material.
The phone calls were pored over in great detail in the ws's of AE and JM, JB's police interviews and trial documents but not one word about an AM because it simply did not exist pre murders.
Whether you believe JB innocent or guilty he will not be a reliable witness circa 40 years later. Even if he wants and believes he is telling the truth he is unreliable by virtue of what he has been through during the last 40 years.