Spray with a weak solution of soapy water using washing up liquid.

Just do what mother nature will do and plant a tagete plant next to the lettuce and the green fly wont want to know,
All this using washing up liquid etc is just giving the green fly a nice welcomed shower, they'll be back before you know it & with the rest of the greenfly family,
they''ll proberly have a green fly bbq after their soapy shower,

Do what mother nature doe's and you'll get a result that works while your not there.

Thanks very much for your comments they were very helpful