The way I see it is this.
You have a Windows Service, which is playing the role of a scheduler and in it there are some classes which simply call the webservices and put the data in your databases.
So, you can use these classes directly from the WebUI as well and import the data based on the WebUI trigger.
I don't like the idea of storing a user generated action as a flag (trigger) in the database where some service will poll it (at an interval which is not under the user's control) to execute that action.
You could even convert the whole code into an exe which you can then schedule using the Windows Scheduler. And call the same exe whenever the user triggers the action from the Web UI.