Thanks for your suggestions Sean. My own comments seem to be getting stuck in moderation, which makes the conversation rather stilted!
I eventually read the right manual, and have successfully implemented your first suggestion. I don't have any licence as far as I'm aware, since one is not necessary for using Quartus web edition with Cyclone V.
http://www.altera.com/download/licensing/setup/lic-setup-q2webedition.html tells me:
--- Quote Start ---
Beginning with version 8.1, a license file is no longer required for this or future versions of Quartus® II Web Edition software.
--- Quote End ---
It seems the same does not apply when using OpenCL with the Cyclone V?