Who says Jeremy Bamber stole diamonds from Mabel Speakman's safe?
Why wasn't he charged with this, along with the murders, given that it was such a serious crime in its own right, with (I assume) no involvement from Julie?
If he confessed to the caravan park break-in, why not confess to this too?
If Jeremy stole the diamonds, then why wasn't he charged? Surely a prosecuting lawyer would be jumping up in the air and doing high fives with the police on hearing about that.
Was the crime reported to the police?
Was a claim submitted with Mrs Speakman's insurers? Even if she was dead when this was discovered, the insurers would cover the executors of her estate.
If the answer to those questions is 'No', then why should we take this seriously, if nobody else did at the time?
What would there be to stop the relatives simply making all this up, as a further way to discredit Jeremy and blacken him more than necessary?
Let's take a different tack and assume that Jeremy did steal valuable jewellery from his grandmother, then why do you think Jeremy decided not to stage a break-in or even actually break-in? Why cast suspicion on himself by taking them without any staging of forced entry? He must have realised that the jewellery would be missed eventually and, in the absence of any signs of forced entry, it would be a suspected 'inside job'.