Thanks to everyone who posted re the speedplays - I think, like CR400, I'm a little wary of my feet getting carried away with themselves when climbing and standing out of the saddle, etc - but it sounds like it's just a question of adapting and getting used to the feel of the pedal.
I'm a bit pigeon toed on the bike (sadly without the wings, to make me FLY!) so I think perhaps the X-Series would be better than the Zeros.