I can't add another link back to the main artwork table as I have too many.
It is important to distinguish between tables and table occurrences. "A link back to the main artwork table is always possible", but a link back to a given artwork table occurrence may not be, though "too many" would not describe the reason why.
You'll need to describe what is not working in more detail. You are on a layout that specifies what Tutorial: What are Table Occurrences? (box in the above relationships graph) and with a portal based on what table occurrence?