Here's a cracking solution for killing greenfly... cigarette butts!

Save your cigarette ends, add some water, leave over night, sieve the mixture off and into a spray bottle, spray your roses with it.
Job done, next day, no more greenfly!

My usual tactic also works, washing up liquid with water in a spray bottle, seems to seal the greenfly in, they don't recover.

Either way, it's a win-win on greenfly.