Could try 12% peroxide gel designed for hair coloring under food covering "cling" film out in the sun. It's just another (highly effective) form of bleach. Get some good masking tape like the plastic duck brand masking tape and tape this film or a cut up food freezer bag around stain on 3 of 4 sides. Leave it 30 -45 minutes, clean off and reapply.If you let it bubble out from your film the peroxide will dry and your stain is simply moved. A pipette if you have one is good for injecting the peroxide gel under the film
I read somewhere that UV make the peroxide 30x as effective. Do not mix this with Vinegar or Chlorine bleach. The former makes mustard gas and the latter will foam up and could explode in your face...!