You can spray with soapy water if you use something like Ecover environmentally friendly washing up liquid. Don't do this though if you use the normal liquids. Te other thing you could try is a garlic spray, though I am not sure it will work if the aphids are already on the plant.

Spray them thoroughly and repeatedly with a solution of organic dish soap. That's non-toxic to you and your friends, but it has no residual action, so any blackfly you miss with the spray will survive to repopulate the plant, often within a week! Be sure to lift up the leaves to spray the undersides, too.

Try and find some ladybirds to lay their young - they'll eat them in no time - they did on my runners and they are all gone.

Snap, MG! In my experience, spraying garlic once the infestation is underway just makes them think they grew up Italian.

Ok, I've tried the garlic spray. If that doesn't work I'll get some Ecover. It is one plant out of twelve so I didn't want it to spread.
Thanks everyone.

After the garlic treatment it is me that spells like an Italian salad.