Many thanks for your reply feanor.
I did try using gps visualizer to create a gpi file from a gpx file but must have screwed something up. I ended up with a list of things just called waypoint and a number and for some reason all of these things seemed to be several thousand miles south.*
I did establish that my Etrex20 seems to be able to accommodate more than one GPI file though.
So I dug the laptop out, momentary panic when I couldn't remember the pasword, ran POIloader, which is indeed very fast and smooth. Used a thumb drive to transfer the gpx/gpi files back and forth from chromebook to laptop and to Etrex.
I decided that in practice it might be best to just have a single file anyway - no great problem to download the GPXs again and it prompts me to make sure that I am using the most up to date files on a trip.
So thanks again to Ben T for this.
https://yacf.co.uk/forum/index.php?topic=95785.0meaning I have the latest list including the rather nice sounding recently opened Whalebone in Downham Market (appears to have a nice garden) and the latest collection of Archies campsites for the UK.
So, for now, will still be resorting to the cranky windows laptop.
Thanks folks.
* this was the spoons - horreur!