Recently I changed my physical residence. Since my ISP serves both residences, I was able to move my internet access over without problem.
The move did however, cause WinSCP to be unable to connect to my web hosting site ie I was unable to connect to my web server via sftp using WinSCP.
The problem turned out to be that the Web Hosting company has to open a "hole" in their firewall for each ip address that uses SFTP. When I changed my residence address, the dynamic ip address machinery gave me a new IP address. The problem was resolved by contacting the web hosting company and getting them to open a "hole" in their firewall for the new ip address.
I hope this helps anyone in the same situation. With my ISP there are two levels of service, one with dynamic IP addresses and one with fixed ip addresses (much more expensive). So if you are unable to connect with WinSCP, check that your IP address has not changed and if it has, contact the admin of the target machine and get him to open a new hole in their firewall for the new IP address.
No need for a response Martin, excellent program.