Well after lots of playing around it turns out that by using the Phottix transmitter off camera (e.g. so I can take self portraits), with receiver #1 cabled to camera's sync port & receiver #2 into the Speedlite, there is an impact on firing.
Looks like flash fires before camera (since I just cannot capture the flash regardless of reducing camera speed).
IF I put transmitter in camera hotshoe (so just have 580EX receiving) flash fires perfectly and blows image.
Guess I'll have to rethink the off-camera triggering