Is the machine on (no power failure)? Is sshd running on the correct port? Is the machine able to establish outgoing connections? Is the router turned on (maid knocked the plug out)? Is the router able to make outgoing connections? Is the IP address or dynamic address you are using correct?
All the above checked and correct.
I sat next to the PC and tested via 3G on a laptop.
If you changed to port 25, how did you change the daemon to listen on port 25 if you can't get access to the machine? Port 25 will work outgoing of course, but I don't know if it is one of the "blocked" incoming ports if you have enabled "ADSL security" on your account. This blocks port 80 incoming, amongst others I think.
I did some port mapping on the router (Billion Virtual Server feature). When that didn't work I just did a straight mapping of SSH. Still doesn't work.
A tcpdump on the interface on the PC doesn't show a single packet coming through and there is no firewalling on the PC (Linux).
I'll set up a PPPoE connection on the PC tonight and do some more debugging to eliminate any sort of firewall issue on the router.
Thing is I've done this before (I do networking security for a living) and it's a piece of cake.
If I have no success using a direct PPPoE connection then the traffic is definitely being blocked at the ISP level.