The low cost model means using as few crews as possible to run each days programme. To give the middle duties to allow you to go from lates to earlies you have to split the working day into 3 duties (early, middle and late). This means you then need 3 crews a day rather than 2 if you just have lates and earlies which is an increase of 50%. There lies you problem. The only low cost way to get round this is to do blocks of lates and earlies i.e. 5 earlies 2 off 5 lates ect.... This is what is done at easyjet but it required special permission from the CAA. My understanding is that the CAA are extreemly reluctant to grant this to anyone else...............