Hi Victoria , just caught up on the thread
Shame about the drainage issues, do other allotment plots have same drainage issues ? Have you consider a bog garden / area if you cant solve it. That clay looks awful to dig, your going to need all them bags of manure.
My Sedum don't seem to attract as many bee's and butterfly's as I hope they would but i like the flowers anyway so wont get rid. I would high recommend a Eupatorium for Butterfly's and bee's, they have a feeding frenzy on mine, they are quite large but I am sure there a smaller one called baby Joe.
Best of luck next year