In reply to girlymonkey:
> but I have also heard that soapy water (washing up liquid diluted) sprayed on them is also meant to work. I have no Idea what concentration though
Works for a lot of insects, but works really well for aphids.
Roughly the weakest mix that you could blow bubbles with using one of those kid's little plastic bubble blowing kits.
What you are looking for is a mix that will just about hold a soapy film. This blocks the spiracles (breathing holes) of the aphids and they suffocate.
Use a fine spray and really soak the plant.
First spray will remove 90%+ of a serious infestation, but you need to go back either each day or alternate days until everything is dead, otherwise a few surviving young will rapidly turn back into a major infestation.
Doesn't really harm any other insect that wasn't on the plant when you sprayed, so your local pollinators should be safe.
Never seen it harm any of the plants. In fact once the aphids are dead the plants really bounce back quickly.