Without identifying a crime scene it will remain non-existent. There was no evidence of a crime in the lay-by. Witness accounts of 'a car similar to Janice's' doesn't mean it was her car. How many of the witnesses came forward after seeing pictures of the actual car in the papers? The only physical evidence is her car parked in London with her blood on the inside of the windscreen and her body found dumped by a lay-by. Despite anniversary appeals, there doesn't appear to be any new forensic evidence in this case that we know of.
I think the purchase of the new tyre is crucial, as is the grease under her fingernails, the missing tyre, and the tyre iron murder weapon. They all suggest an auto garage to me as the most likely murder scene. Someone with control of such a facility, closed Saturday afternoon and Sunday, would have ample time to move the body, then move the car, and clean up the scene before opening on Monday.