You need to change the source code and rebuild the payment method
So I have found this in a folder called "nopCommerce_1.7_Source Folder" that is outside the root folder on my computer, but I am unable to access this via ftp on the remote site. Is there somewhere else I should be looking for it in the root folder? What is the breadcrumb trail please?
Source code files really should not be on the site. You need to have them local on a developer's PC. You will need Visual Studio to rebuild the plugin dll.
The source code is compiled into executable assemblies (e.g. .dll files), by a tool like Visual Studio. The executables run on the site.
The exception is the .cshtml files ("views") which will be compiled just-in-time by the ASP.NET. I.e. You can use any text editor to edit .cshtml files. But in your case, "The url is hardcoded in the payment method ", and you need to rebuild that project.