If someone can give me an answer then I am happy to wait to another couple of days.
I can only assume that the wall was brought down by the additional construction and plants that my neighbour allowed to grow upon it. The wall was old and weak and would have fallen down eventually, but perhaps not for many years if it had been maintained instead of having placing strain placed upon it.
The additional dense ivy and grape vine grown from the neighbour's side towered well above the existing wall and surely presented a much greater face to the strong wind. Ivy is also known to damage mortar by sending roots into it. I took many photos of the downed wall showing the extend of the vegetation and the attachment to the wall of additional structures.