So, tried both workarounds suggested here:
https://support.displaylink.com/knowledgebase/articles/1181623
with no joy.
I also tried live cds of different distributions - turns out linux mint 18.3 works better (not perfect - there are cursor trails at times, but I can live with that...) so I've created a bootable usb stick (not live) which I can use, and which works with the DisplayLink monitor. I'll spend some time with it before making a plunge and installing a new OS to my laptop. In the meantime perhaps someone will come up with a fix...