Thanks for your enquiry.
If there is nothing in either set of Deeds showing that one party is responsible is responsible for maintaining a boundary wall, then the law states it is joint responsibility/a party wall.
Hence, unless your neighbour can prove, by way of documentation, that this wall is your responsibility, it is indeed a party wall!
I hope this clarifies the position to you.
If I have helped, I would be grateful if you could rate my answer.