Before getting too involved in making my own build - although I did look at DiapDealer's remarkably clear instructions - I made a visit to the Sigil flatpak's github site.
I posted an 'issue' about the matter of difficulty for the plugin to get in touch with java. I also mentioned ODTimport which also needs java.
Overnight, this issue has been taken into account with an update pushed. The Readme has also been updated to state :
Quote:
For plugins that need java runtime
flatpak install flathub org.freedesktop.Sdk.Extension.openjdk17//22.08
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I used that line of code to install openjdk17 and installed the updated flatpak version of Sigil 1.9.20. I tested this satisfactorily with my epub file and EpubCheck.
I havn't yet had a chance to retry ODTimport - but I am optimistic.