There was this animated film i saw years ago with these black humanoid figures that disguised themselves as humans. The only real difference between the two was the aliens had heads where their crotches should be and genitals where their heads should be, and they spat into eachothers mouths as a greeting. The main character is disgusted by this and finds his society being taken over by these creatures, and at the end he finds out his family are all aliens and kills them, only to find his own head fall off and reveal the genitals beneath.

The animation was quite simple with few colours - i particularly remember the black figures and dark orange backgrounds.

I'm sure it was called Zogs and narrated by Bill Bailey but i can't find anything on google nor Bill Bailey's listed bodies of work.

I must have caught it late one night on regular british TV.

Does anyone remember this film?