Further update.
I have downloaded and installed wxwidgets 3.1.3 from source thinking Audacity 2.4.2 would work but am now getting this error message when I run from Terminal.
audacity: symbol lookup error: audacity: undefined symbol: _ZN12wxWindowBase14RegisterHotKeyEiii
I can see that there are patches to do with Audacity in WxWidgets and also that it needs to build against version 2 and not 3 of GTK but I don’t know how to get or build that version.
SOLVED:
Followed the instructions here, using the latest build and remembering what I did last time with Steve’s help.
https://forum.audacityteam.org/t/building-audacity-2-4-2-on-ubuntu-20-04/57719/1