I do hate replying to my own post but I thought I would post here how I fixed my problem in case anybody else ends up here with a similar one.
To be clear, I have 2 monitors and when I enabled transmit the full screen playback would be overlayed over my Premiere workspace on monitor 1 even though I had specified monitor 2 as my video device in Preferences > Playback.
It appears that at some time in the last few months (I have a vague memory of this) I had decided to check out the 'improvements' in the way Mavericks handled multiple monitors. I created a couple of spaces in Mission Control at which point the Applications in the dock gained an extended OPTIONS menu on the context menu (the one that pops up when your right click on the app icon in the dock). I knew this extended menu was there because the Apple website told me so.
The extended OPTIONS menu on my 2 monitor setup now contained ASSIGN TO > ALL DESKTOPS; DESKTOP ON DISPLAY 1; DESKTOP ON DISPLAY 2; NONE.
After trying out the different options to see if the operation was consistent with Apple's explanation and whether I wanted to work with spaces I decided that I didn't so I removed the spaces I had created in Mission Control.
Removing the spaces removed those extra ASSIGN TO menu options although I wasn't aware of that at the time.
A few days later I went to enable TRANSMIT when editing in Premiere Pro (which I had used successfully before) only to have the problem described in this thread. I stupidly credited this problem to a recent CC update and when the CC 2014 update didn't correct it I started to look elsewhere.
Cut to the end and the solution - - after proving that a new test Mac user account did not exhibit the problem I returned to my user account, enabled some spaces in Misson Control, changed the ASSIGN TO menu to NONE for Premiere Pro (the default) and then the TRANSMIT function worked correctly, the full frame playback was assigned to the correct window.
Ok, the last paragraph may suggest I knew what to do but I didn't, I had spent a lot looking in the user library to look for differences until I stumbled across this.