Our cabbages failed for two reasons last year. Firstly the ground is lifeless, having had nothing added to it (except the terrible compost they sell here perhaps) for years and secondly, the few cabbages that went well got eaten but caterpillars. So I'm going to give this a try but modify it a bit by putting the images back-to back, so that if it blows over in the wind the image can still be seen.
Going to buy cabbage plants specially to test this decoy.