You can buy liquid soap from the Organic Gardening catalogue, either ready mixed in a spray or possibly concentrated (don't know if they still do that). You can also use very plain soap such as Simple and rub some into water - so it becomes slightly opaque, or you could use Lux flakes if they still make them, then use it in your own garden sprayer. You need to cover the entire insect, and you may well need to repeat the spraying every day or so for a few times, and remain vigilant. Not being a poison it won't have a persistant effect, but don't spray it on beneficial insects as it could affect them, depending on type. The only aphid I have found this not to be successful with is Lupin Aphid, which is bigger than most aphids.