Right! Not how I remember this working, but it works and is simple enough!
Refering to this 2011 blog post, i downloaded a template python script tool from here which is summed up in the screenshot below.
The script tool uses arcpy to set the input features, field and field value. In my use case, I only have the Field value set to be selectable from a drop down list (parameter) but you could set all 3 and the field and value would be dependant on the feature class selected. The output Value is then used in subsequent GP tools for selecting data (in my case, select by Attribute and Append).
Quite a simple solution in the end! If I find the other solution that I believe also achieved this, I'll share that here too.
***EDIT***
I removed this as the solution because it seems to be a bit glitchy. The first time it ran, it did generate the list, but then that list never seemed to update for subsequent runs as new values were added to the feature class. Shall keep digging!
Lindsay Raabe
GIS Officer
Forest Products Commission WA