I've used washing up liquid in a spray.

Soft soap (made using caustic potash instead of caustic soda) is much
better. I think Chempak is the name of it's manufacturer's. Restrict the
spray to the massed aphids for it will not protect. It will also help to
kill of the ants with something like Nippon for they protect aphids from
ladybirds' attacks.

Geoff