Thanks very much for your detailed replies. I've tried pulling down and up the sizing bars but to no avail, please see screenshot:
https://drive.google.com/file/d/1cBGRLi1MovejlEhWkmn1_7kNpwX339Hg/view?usp=sharing
Also, I'm a bit confused why you'd need multiple video tracks (apart from text overlays), because presumably only the top video is visible in the final video and I assume it's not possible to do transitions between video clips on different lanes, or perhaps I'm missing a technique here?
Thanks again for your help,
Jeremy.