Apart from the recollection of the man who took the order for the plate, what is the evidence of the transaction? The incident was supposed to have occurred only hours after her body was discovered, in a nearby village, and before her car was found in Regents Park. Doesn't make sense.
The man didn't ask verbally for a registration plate but handed over a piece of paper. Does that piece of paper survive as evidence? Was a receipt issued with a counter foil recording the plate number? Presumably, he paid cash. Bit risky asking for a replacement plate of a murdered woman's car so close to where the body had recently been found, with her car found 60 miles away without new plates. Either it's just noise in the investigation, or on the outside an accomplice or the Perp sowing confusion. I fail to see how ordering replica plates at that point would do anything other than draw attention to someone, or perhaps draw attention away from the person ordering or committing the hit. And I do think she was executed to order. Who benefited? He was interviewed but had an alibi. MOO