An FoI request to ask whether the pothole had been reported prior to your incident may reveal that the council had prior knowledge of the pothole's existence and I believe that would be the proof you need to sustain your claim in court.
You should also ask for a copy of the annual inspection report on the state of the road where this happened, to see if it turns anything up.
A word with some locals is not a bad idea either. It wouldn't surprise me if one of them hadn't already complained if the pothole is THAT big.
