If i'm not wrong a document is unique to each doc library. What i want to do is use some fields the users select to make up a document name.e.g docname_Created I'm sure i can do this via Object Model but can i use a SPD workflow for this? Any ideas would be appreciated
You'd either need to write an event receiver in visual studio or use a third party product as described in this example in combination with SharePoint Designer.
You're much better off populating some metadata fields than cobbling together some sort of compound filename. You'll just be replicating the poor practice of overloading the filename with significant data that can't ever really be teased out reliably.
There is a Title column that you could use to write your name to. I would opt for a Calculated Column myself and expose that in the view. Is there a reason you need the Name column to be updated?