Hi Graham,
To access your FTP server from a remote location you need to connect to it via your external public IP address provided to you by your Internet Service Provider (ISP). At least, without more information regarding your host and specific error messages, this is my first guess of what is wrong.
Consider a following example:
Our FTP server is running on a local host with a local IP address say 10.1.1.30 and port 21. Any host on the same network id 10.0.0.0 can access the ftp without any issues. However, if we try to access it from a remote location we are no loger on the same LAN as out FTP server hence we need to fist connect to our router via external public IP address e.g. 23.45.67.89.
In order to do so we first need to setup port-forwarding so any remote attempt to access socket 23.45.67.89:21 will be redirected to 10.1.1.30:21
If you have not set your port forwarding yet login to your router and look for a feature port-forwarding.
If still unsuccessful paste here any error messages you encounter when trying to connect from your office using filezilla. Alternatively run command ftp command to attempt to login from your office and paste the output here. Example:
$ ftp my-ftp-server-host-name-or-ip-address
In any case more information is required to in order to troubleshoot.
Lubos