Small postscript to this - I've experimented with assigning the WinTV-CI to the AzureWave card as its external CI module. This works, provided I use my T-Rex cam for the BIS card.
I've ordered an extra T-Rex cam and another WinTV-CI module to see if I can run Irish
Sky, UK Freesat and BIS together. If I get all this lot working that will be it for the Dreambox for me!