None of the avalibale servers could be easily be set up to authenticate the way I needed. Most of the other servers required root access to install or run, which was not an option.
Many servers suffer from grave security problems.
So instead of working out how the other ftp servers worked in order to change them, I wrote a new one. Over the last 9 months, I have added most of the features required for a general use ftp server. Fast and light-weight. Runs with or without root access. With root access, it can act like a normal ftp server.