I use soapy water and also remove any badly infested leaves. I also find if one of my plants is very dry the blackfly seem to go for it.

If you do want to use diluted washing up liquid, it should be an eco friendly brand - using ordinary washing up liquid is worse than some pesticides. You will also have to use it virtually daily because it has no residual effect, only kills the ones it touches. You may have been using a contact insecticide, which again needs repeated daily treatments. Alternatively, go out and buy a systemic insecticide which treats for aphids - spray the leaves and stems all over (not while they're in sun, preferably at the end of the day) till run off, and then repeat in 10-14 days if the problem recurs. Note that systemic means it enters the sap stream and remains active for up to 14 days - contact means it works on contact with the insects only.