Turns out that file that I write to using FPUTS onto N:\TIMING\DIBS\A4FILE.TXT was the problem. Somehow the opening of the table was a red herring. I move the location of this file to C:\TIMING\DIBS and it works fine.

The file N:\TIMING\DIBS\A4FILE.TXT is Read/Write/Shared and the user has full rights to it?