Thanks Jason - I've made a few changes to the database structure to try it out and have the drop downs working nicely on my add and update keyword pages as I would like. So most of it amended quite painlessly.
The one thing I haven't gotten quite right though are the behaviours on the <body> tag to get the two dropdowns to display the current values of the keyword being updated.
I have attached the page as it is, with more details below.