Just a follow up Marc. This is very bizarre. I moved the page to a different server (which should not matter as I successfully use GetListItems there anyway). But in the new location I now get a error code 0x81020020 Invalid URL parameter. So like you say it must be with the site name. This site name is the exact same one I successfully use with GetListItems. Fiddler confirmed the name / lists / traffic.
I am going to try and create a list elsewhere and report back to you what the fix was in case I can solve it.
