Thank you Sacha, it would have been a good idea. I tried to get the
phytoseiulus persimilis and they came all dead in the package through
the post.

Besides now I had I already sprayed with Imidacloprid and Dicofol and
I'm afraid now it's may be too late to use organic controls.

Now, following Kay's advice I wash the plants leaves with a solution of
dishwashing liquid and water every other day and I spray with
insecticide once a week. They have reduced in numbers but they are
still there I think... Some leaves still show signs of attack.
Maybe I should spray the outside of the pots and the area around them
as well...

Humidity and cold weather have reduced their number considerably. A big
thank you, Bob, for your help!