Thanks for the very quick reply. I realise that I failed to copy the first line of my post from the the text editor where I was preparing it; the first line said 'PhotoMechanic 5:'
Sadly, this version does not have 'Ingest from selection', does it ?
<Edit>
Apologies for asking the question in the first place: I now see that this topic was covered extensively in the forum 11 years ago - I'm just a bit slow in catching up!
I also see there is a work-around using locked images and have now discovered that, rather than setting the lock on an image in the camera, it is much quicker to browse the SD card (in, say, Windows explorer), select the images for the 'first' theme and set their file properties to 'read only' in one go, go to PM and ingest - choosing 'locked-only' images to ingest, complete the 'first' ingestion task, return to Windows Explorer and reset the locked images to clear the 'read only' flag in file properties, then set that flag on the images for the 'next' theme and repeat the ingestion cycle as many times as necessary.
A bit unwieldy, but it works exactly as expected.