When referencing the parameter, try using .count instead of .value. It is possible to compare this with the 'count' of records returned by the query used to populate the paramter...

Solution works perfectly, thanks!

Identifying the user is NOT the problem

Thanks for the response, Hari

Identifying the user part isn't the problem.

The problem is how to enable/disable the drill-down dynamically and show/hide the +/- toggles appropriately.

I have worked around this, probably...