This question was posted by a colleague but it hasn't received a reply. I'm guessing his TechNet benefits was not configured correctly so I'm reposting it for him.
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I'm using _vti_bin/lists.asmx?op=UpdateListItems web service to update a task list field passing the required infromation but the update does not take place and the web service does not return an error. The ID of the item exists and is the correct item to update. The account used by the web service to make the update has Contirbute permissions on the list. Any ideas what has gone wrong?