Mustard is the answer, sow anytime March-October and it grows fast. It also suppresses weeds and repels a lot of pests. I have even over-wintered it and it really protect the soil from damage. Also adds masses to organic matter to the soil.
Only downside is that it is a brassica so you can't follow with cabbages or swedes in a rotation system.
I haven't tried buckwheat yet, but it has a smaller window in which it can be sown.