Also:

"A 2% soap solution is the quickest & best aphid control. This would
be about two ounces of dishwashing detergent per gallon of water, in a
sprayer. There is no extra "oomph" from paying extra for special
insecticidal soap where aphids are concerned, dishwishing detergent is
fine. Drench the tree especially the undersides of leaves. Once is
usually enough but at most a second time two weeks later. "

-paghat the ratgirl

I read somewhere that ladybugs (which are available in shops in the
UK) and some other insects eat them and that ants rear aphids.
Might deter them in the long term.