In my mind, I have to reverse that image:
This is really the same structure as an Invoicing system where you'd have:
A portal to LineItems can be put on your Stock_complete layout. The _fk_stock_parts_id field can be placed in the portal and formatted with a value list of values from the Stock_parts table. You can either include fields from stock_parts in the portal or you can can define fields in stock_line_item that use looked up value auto-enter settings to copy the data from stock_parts.
Here's a simple invoicing demo created by Comment. If you were to rename Invoices as Stock_complete and Products as stock_parts, it should serve as a simple model of how you might set this up.
Many thanks for that...exactly what we were looking for...just couldn't see how! As I said before we are new to Filemaker, but it is begining to fall into place and we are very impressed...thanks again.