Brian Gaff asked:

Without significantly demolishing the wall, how can you do anything other
than you did?

I spoke to the builders this morning. They say that they can significantly
reduce the amount by which the fascia, cladding and tiles protrude from the
basic wall. They also accept that the initial mistake was their fault and
will do the job at their own expense. This will be in accordance with the
planning department's running: "trimming back" was specifically mentioned.

The exact position of the party wall centre line might not now be a vital
issue: the builders are happy to accept the "assumed boundary line" as shown
on the drawings.

Bert