I have it working, but there's an odd bug which I don't understand, which is is doesn't stop if you unload the script, you have to close mirc to stop it.

To start it after you've loaded the script you type @Trigger and it will respond with the message and url, then every 300 seconds after it will play the channel ad until you close mirc.

Oddly having the trigger in the message doesn't seem to create a feedback loop.

Code:
on *:text:@Trigger:#ChannelA:{
  /notice $nick Latest List at: http://example.com
  /timer 0 300 /msg #ChannelA Type: @Trigger for my list
}


I'm new to all this so it's pretty raw. Any suggestions on improvements gratefully received. Thanks