From Ubuntu 22.04, the solution is to download the gnome tweaks app, go to 'keyboard and mouse' and click the "Additional layout options" button. You are presented with a list of drop-down lists in a new dialogue. Click on "Compatibility options" and select "Enable APL overlay characters". The appropriate font is already installed. Everything then just 'works' - e.g. press ALT-r to get rho.
@Adám: Hi! Thanks for the welcome. I look forward to reading and learning here every so often.
Oops - I should have looked at the UI more closely - more haste and less speed and all that! I'll copy and paste the answer to the thread - unless there is a better way?
@Mark It isn't a thread as such. Stack Overflow is a Q&A site. Put your answer into the text box below the "Your Answer" heading. (Don't post it as a comment below the question!)
Sorry - I meant my version earlier in the chat needed the correction. I have added the extra step to my Stack Exchange answer to the question as well as a link to the APL WIki article.