Greenfly on my leeks

Got a really bad infestation on the leeks in my greenhouse. In fact too many to remove by hand. I guess I can't use an insecticide like the Woolworths Bug Gun my wife has for the roses. Any ideas please? They're starting to look sick.

The Woollies insecticide contains pyrethrins.

I'd get those leeks outside to harden off, if I were you, Scribbler.

Put some soapy water in the spray mister thingy, adjusted the nozzle so it was jet rather than a spray, and blasted the little b*ggers off. Leeks now hardening off and seem to be recovering.