Hi Kitsie.
It is called 'Bloo Toilet Gel with micro crystals'. It states safe for use in septic tanks, and is made by Jeyes. I am sure it must have some bleach in it though, because of the warnings of skin and eye contact though. Another one I used to buy was 'Parazone', also suitable for septic tank systems. 750ml size bottle, and it lasts a long time because of infrequent use.
Excellent idea about denture cleaning tablets - thanks.