French marigolds to see off conservatory green fly

Several years ago I got greenfly in the conservatory and couldn't get rid of them until I tried french marigolds which worked like a charm. Now every year I buy a strip of 6 french marigolds and put 2 or 3 in the conservatory and the rest in the garden where the slugs eat them. This year in B&Q and Homebase they only had sets of 12 marigolds for sale, and giving up 9 or 10 marigolds for slug food seemed a bit much. I didn't buy any, hoping that not having had any greenfly in the conservatory for years would mean for a year at least I would be OK. Then last weekend I had taken some puppetry books I no longer wanted for the Puppet and Toy Theatre Guild stall at the May Fayre at the Actors Church in Covent Garden (every year lots of Punch & Judy Professrs come to this "fayre" to celebrate the first recorded sighting of Punch in England - in Pepys' diary). On a residents association plant stall were some french marigolds, so I brough 2 at £2 each. This was about the price of 12 at the DIY store, but it was money for a good cause and no waste so I didn't mind. I put them in a small planter today. I also put the mint into a slightly bigger plot. Was pleased to see 2 bumblebees feeding from my wysteria.