It would be great if bookings could change from red to another colour when the guest is over 15 minutes late, so that we know we can reallocate the table. 

We would also find this very useful.  We state in our T&Cs that tables will be held for 15 minutes, and we don't consider bookings as arriving late until 15 minutes after the booking time.

Here's a properly written user story, in case the development team wants to take this up:

As a venue, I want to be able to configure the time (in minutes) after which a booking will be considered as "late", so that the use of the "late" (red) arrival status colour in the diary becomes useful to my staff.

Background: Most of our customers arrive within 15 minutes of their booking time, and we state in our terms and conditions that tables will be held for 15 minutes.  We don't consider a booking to be "late" until these 15 minutes have elapsed.  Our door staff would like to be able to rely on the red colour status for "late" bookings to quickly identify those bookings that are genuinely late by our definition (i.e. more than 15 minutes after the booking time).

This could be implemented as a single "lateness" configuration setting for "number of minutes after which a booking is considered to be late".  The booking status should remain as "unarrived" (blue) until this number of minutes after the booking time, after which the booking status becomes "late" (red).  This implementation preserves backwards compatibility with all existing ResDiary deployments, provided that the default value of this "lateness" configuration setting is "zero minutes".

Acceptance criteria:

* A configuration setting for "lateness" exists within the Settings portion of the ResDiary user interface, accepting a non-negative integer value in minutes

* The default value of the "lateness" setting is zero

* The status of an unseated booking changes from "unarrived" to "late" when and only when this "lateness" number of minutes has elapsed since the start time of the booking

* When the "lateness" setting is zero (i.e. the default), the status of an unseated booking changes from "unarrived" to "late" at the start time of the booking.  (This is the existing behaviour that must be preserved for backwards compatibility.  Note that this criteria will be met automatically if the above criteria passes.)