>What I've got is something like this
>
>SET REPROCESS TO AUTOMATIC
>SELECT NextRef
>=FLOCK()
>Read the number
>increment the number
>write it back
>unlock
>
>Am I right in thinking that with reprocess set to automatic the flock() function will attempt over and over again to lock the file until a successful lock is placed? In which case it the code below the call to flock() will never be executed unless the file is locked.

Generally, yes. But now be careful;
Once the Esc button is in effect, and the user can press escape, the FLOCK() attempts will stop. And then the code continues ...

Look at this help snippet too :

Note If FLOCK( ) fails to lock a table, it returns false (.F.) and does not generate an error. As a result, you can’t use FLOCK( ) to trigger an ON ERROR routine.

This would happen at Esc (when allowed), and you really should be coding like this :

SELECT NextRef
IF NOT FLOCK()
  *** some message ***
  RETURN 'Failed'    && Give possibility to try again or abort the process.
ELSE
  Read the number
  increment the number
  write it back
  unlock
  RETURN 'Succeeded'
ENDIF

HTH,
Peter