I read this whilst sat next to a lawyer, so I asked her whether or not there's any truth (in general terms) in the idea that possession is nine parts of the law.
She said that it can still be true in terms of land ownership (which is where the notion originated) but that the idea has very little application for other tangible things.
Then she told me something I didn't already know, which was a change made to the law of Theft in the late 60s regarding possession, ownership and evidence.
In the welter of newly-created dining out in the newly-created restaurants of the 60s, a new scam was developed. People would walk into a restaurant, order a meal, eat it, then refuse to pay for it on the grounds that there wasn't (now) any evidence that they had been served a meal ... the food was all gone.
Under the law of Theft as then framed, this was enough to enable them to plead Not Guilty and to walk free from the Court.
What a great idea. Shame they changed the law really.