Did you try new sequence from clip from the sequence containing the 2 cams in the Project Window?
yes, tried even strange things like duplicating clip, then renaming, crazy.
OK, restart computer, open Premiere while holding down shift+alt.
See if that will help.
That helped, back at it - thanks. New trouble - although I can now make the multi-cam target sequence of the correct duration, I move into program monitor for editing the multi cam display enabled, the separate sources/cams no longer show in the window... it's just blank. I've checked my vid drivers- up to date with manufacturer (not windows) updates.