I'm a novice user of a polytunnel. Will putting fragrant plants in it (such as Daphne Odorosa) attract butterflies that will damage green leafy stuff I'm also growing?
It depends on what you are growing. Here is a list of food plants for some moths and butterflies in UK.
Unless you are growing the cabbage family you have nothing to worry about, If you are then the Large White butterflies will find them whatever else you grow.
Thanks pansyface, the list you gave didn't mention any of the brassicas but I think Alan's point is what precisely I need to know. Yes I'll introduce fragrance into the tunnel and keep the brassicas outside. Thanks to you both.