I was going to stop posting as I fixed my own problem, however the fix is worth recording as it’s not easy and may help others.
PROBLEM : I got the infamous “Libreoffice requires a Java runtime environment” when trying to do Macros and macros in Python with my Mac. I had just installed LibreOffice (5.1) and installed or re-installed a JRE (build 1.8.0_72-b15) but the error appeared everytime I clicked Tools > Macros > Organize Macros > any option. Even after quiting the program.
SOLUTION : Luckily I noticed that the Tools > Macros > Record was disabled; so first I searched the Net to find out how to enable it - this was also not easy again. The option is hidden under top left of Mac screen > Libreoffice > Preferences > Libreoffice > Advanced
I checked “Enable macro recording (limited)” at the bottom. Then I noticed that “Use a Java runtime environment” was checked (good) but that none of the two listed JRE options were checked.
I then checked the latest one (1.8.0_72), pressed OK and then was able to run macros.