How do I add a text field that links to the taxonomy_index table When I add a taxonomy tag to a node, I also want to be able to add 1 (or more) text fields specific to that tag and that node. I know how I would set up the underlying db table (compound foreign key of nid and tid with a many-one join to the taxonomy_index table, a primary key and a text field). 

How would I implement this? Or does a module already exists that has this functionality?

I am fairly new to the world of Drupal so unsure if this would require writing a module, adding a new content type or another approach entirely.

Many Thanks,
Pete
Use the field collection module.

Create a new field collection field attached to your content type, and then in your field collection put 2 fields: 1 taxonomy term reference field, and 1 text field.