A quick (but I suspect not very simple) question:
does a zero fare excess get issued by replacing the existing ticket or issuing a separate supplementary one?
I used to clearly mark the original ticket as having been excessed, because some people cottoned on to the fraud possibilities using excess tickets.
Can you excess more than once?
Say I have a ticket to Birmingham Stations. I excess to University (my origin is far enough away that it's a zero excess) but then later decide I actually want to go to Selly Oak. Can I zero excess again?