especially "bcc" and "brl.mod" have sometimes a hen-egg dependence when updates of them happen
Understood. Although I do not mess with existing modules, I had not updated anything for about a year and since it all still worked okay, felt it wasn't necessary to do so. However, now I can see how even changing the bcc can impact on that interdependency so it is still important that I keep the core 'parts' up-to-date.
I can report that all is good now::
Latest version downloaded from blitzmax.org.
*All non-standard mods removed entirely*
Replaced all the contents of all the (install directory) folders accordingly with the content from the downloaded archive package
Replaced bcc.exe with the 'fixed' version (( as well as copying the bcc folder contents from the fixed repo branch too ))
bmk.exe makemods -a -r pub
Was successful
bmk.exe makemods -a -r brl
Was successful
tested with a project that does not use any 3rd party mods and it did seek to compile the BRL mods again, but did so without error!
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Phew!
Finally, panic over

Thank you so much for all your help!
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At a later date, I will re-add the bah modules and then only compile individually what is required. If there are issues with specific ones, then at least I'll know what's what!