“party fence wall” means a wall (not being part of a building) which stands on lands of different owners and is used or constructed to be used for separating such adjoining lands, but does not include a wall constructed on the land of one owner the artificially formed support of which projects into the land of another owner;
“party structure” means a party wall and also a floor partition or other structure separating buildings or parts of buildings approached solely by separate staircases or separate entrances;
“party wall” means—
a wall which forms part of a building and stands on lands of different owners to a greater extent than the projection of any artificially formed support on which the wall rests; and
so much of a wall not being a wall referred to in paragraph (a) above as separates buildings belonging to different owners;
and of course there is very much case law the original and most famous being Watson v Gray (1880)
Most importantly you also have Bruce’s Cyclopedia of Party Walls
Agreed Party Wall Surveyor serving Thanet
Party Wall Third Surveyor serving England and Wales