Thanks for the reply Fumigator, by 'not working' I should have explained that the UPDATE event query updates the complete field with a blank value.
Removing the [event_id] and setting the complete field to equal $eid returns correctly the $eid value of the highest id. In my case 4. So the last row of the db is updated with the complete value as 4 (which is correct as the ID of this row is 4)
From this I presume I need a way to make each textbox a unique ID that I can then refer to when I post?