Ok, I give up.. I've had people saying there's a problem with "the script" and Node-Red. Turns out NPN 8 can be an issue (my working clones use npm 6.41) - so I used "the script" myself and sure enough... I even manually updated Node and used Dave CJ's script to updsate/upgrade Node-Red/Node.. still wiping nodes out. For now I'm advising people to unselect NR from "the script" and do their own thing. Is there a single place where there is definitive advise for RPI - assuming starting with standard RASPBIAN FULL (with or without extra desktop stuff) - how to get fully up to date NR and for example SERIAL NODE I2c and SQLITE nodes... I think those 3 are also part of the problem along with Node 8 which has a habit of appearing during upgrades.
If someone can advise I'll redo that section of "the script" to incorporate DaveCJ's script and whatever else is needed. Until Node 8, the only (easily resolved issue used to be the serial node but now I've just lot all extra nodes again. There has to be a better way.