Just to update...
I was able to get these in the end... when I went into store, and instead of again trying to select the Premium pack (which continually said I already own it... close?), I selected the China Rising pack on its own, where I was then prompted with "You already own this, would you like to re-install?" After selecting Install, again I got no visual confirmation of anything happening... however I just popped home for lunch and both had commenced downloading. Second Assault has compelted successfully, but China Rising had stopped and I had to again re-download to bring down the final few percent.
This was in no way intuitive and certainly added a great deal of frustration. I hope developers can make common sense a priority in fiture titles/updates.
