A quick Google suggests that although can be broken down by sunlight it could take a lifetime or longer to biodegrade so I wouldn't want it in my garden.

Some of the modern ones are plant cellulose based even though they look like polystyrene, and they dissolve when wet

The packaging I have tells me I can either dissolve in a bucket, or put on the compost heap where it will dissolve, like flour paste I assume

Thank you for your answers I think I will dustbin it all not worth the risk.