You can access any ftp server regardless of port number, as long as you know what it is from any IP static or dynamic IP? That would always be the case.
You'll need someone a lot more conversant with this than I to explain why you need a static IP to access your own server. I was thinking you were using O2's ftp servers. The only thing I can think of off the top of my head is that when at work you're on the same NAT addresses and when accessing externally you have to go via port 25 as the address you want to go to is ftp://xxx
therefore their system does not recognise the address unless it is concluded with :25 once routed to your office the port number may be ignored and just the ftp://xxx
part is used. Providing their firewall doesn't block transit Hence the static IP.
This is just conjecture on my part as I haven't tried it.