I did it! (I think).
It seems if I select the "Audition 3 Windows Sound" driver under EDIT>AUDIO HARDWARE SETUP, the UA-5 inputs still appear as an option on the drop-down Inputs menu.
What has been catching me out is that the converse is not true: i.e. if I select the UA-5 driver, any other outputs do not appear on the drop-down Outputs menu.
I'm now getting a message that says that the driver I'm using does not offer the best quality but I'm going to experiment for a while and see how it works out.
Thanks for the pointers.