Thanks very much for your responses so far!
I'm trying to set up a single alert in a basic Sharepoint site that is set so that only "Existing Items are Modified" and "When a Task Becomes Complete" generate alerts.
Does anybody have any idea what field Sharepoint alerts need to see modified so that it recognises it as complete, as for me both Status & % complete don't seem to be doing the trick.
Cheers - Rob