Bert Coules expressed precisely :
He also claims that the entire construction is closer to the boundary than is
shown in the drawings and that if it had been built in the right place then
the overhang wouldn't have happened. The builder is calling tomorrow to talk
about this aspect.

And that is the situation in a nutshell.

The methods of determining the mid point of the two properties, all
seem awfully complicated.

I would guess that all that is needed, is to measure the distance
inside to the shared wall, allow a bit more to get to the middle - out
to the window, then measure the same amount from the window along the
outside.