It will be a standard size, it's probably damaged. I would get some mole-grips on it and a spanner on the adapter, it shouldn't take too much effort to unfasten it.I just popped out to check the only spanner that fitted was the 1/2 inch, but it slips. The 12mm too small and 13mm too big.
Typical it's not standard!!!!
But watch the pipe doesn't start turning with the nut, that'll ruin it.It will be a standard size, it's probably damaged. I would get some mole-grips on it and a spanner on the adapter, it shouldn't take too much effort to unfasten it.