I think I forgot one step.
https://support.displaylink.com/know...r-only-one-mon
running the kextcache command in recovery mode. Have done that now, including installing the new version. Kextstat now displays:
174 0 0xffffff7f83de0000 0xb000 0xb000 com.displaylink.driver.DisplayLinkDriver (5.2 (48)) A3EFB07A-5F72-3214-8C15-5F645B87E0CF <114 13 6 5 3>
which says version 5.2 driver. Will be testing tomorrow when I'm back at work.