Thanks for all the replies and sorry for the delay (my son came home, so it's fairly manic until he's gone down). I think the overlay answers cracked it - there seems to be two ways of overlaying - either a theme or a video/still. I wasn't aware you could do the latter, so I thought overlay was for onscreen graphics / effects (like a filter etc) not for adding videos on top of one another. Now I've found that (thanks to you guys!), I can split the video where I want the cutaway/overlay to take place and overlay the video I want then split again at the end of the overlay and return to the original - all the while, keeping the original clip's audio running.
Apologies for not answering everyone's questions directly. I'm struggling to use the right terms for everything I'm doing and the jargon is what's stopped me here. Thanks again.