I have installed it (several times) but it still refused to either
a) work
b) show in the list of installed plugins
in case a, I click on hide tempratures and next time I start the printer the screen it is un-ticked
in case b, I can't see it under installed or available filters.
I have installed it using the url, and directly on the pi using .oprint/bin/python setup.py install.
Both show complete with no errors.
Yep, it's because you have both ActiveFilters and ActiveFiltersExtended installed. Try unisntalling ActiveFilters using the following command and then restart OctoPrint.
cd ~/oprint/bin
./pip uninstall OctoPrint-ActiveFilters
That did it, AF wasn't working so I thought I would try your one. BTW you didn't say I had to uninstall the other one first not could I find any details on uninstalling a plug in.