Soapy water for greenfly?

I have no end of plants infested with greefly :( On a GW I watched recently, Monty Don said he couldn't see the problem with them and they rarely cause damage. He clearly doesn't get the same sort as me! Twisted and distorted leaves and flowers all over the place!
Anyway, I have an insecticide but don't like to use it, so am trying the soapy water method. This is my third day of spraying and they look as plentiful as when I began. How long does it normally take to get rid of them? I'm giving the plants a good soaking every evening.
As a slight aside, why do greenfly attack some plants and not others? My linum and geranium have none, while plants right next to them are covered.

 

Daisy. you could actually be over-doing it. You must give the soap time to get sticky. so don't spray every day.

why do they like some plants? I assume that it is the same reason that not all kids like cabbage.

 

Ah! Right, I'll lay off until it's sticky then. I'm just using washing up liquid in water, is that right?

 

I have tons of aphids - my lupin aphids cause the most damage. i tried my best with soap and water for ages on them last year but they are mutants and I had to resort to chemicals in the end. It worked well with my woolly aphids however and they never came back.



Editing message to say just looked at April tree and woolly aphids are back! - Ah well it worked for about a year, I will be back on a ladder with soap and water tonight