From an engineering perspective I assume they need to filter the recycling anyway, even if your council does sort-at-source. Otherwise a single misplaced item (by non-expert home users) would invalidate an entire batch and the yield would be unacceptably small. ...so if I'm in doubt and the item is clean then I recycle.

This question works equally well for plastic bottles and philosophical theories. What mistakes of the past are we failing to recycle away, and which have we already recycled that we aught to have kept? For instance, fashion in 1983 vs 2018. Perhaps we're doing a bit too much reusing and not enough recycling?