Autumn sowings of green manure mostly fit into the 'prevent goodness being washed out of the soil'. If you've spent all year working soil, adding goodness to soil and cropping soil then soil left bare can have the majority of their goodness washed out by pounding winter rains, this can also leave the soil compacted.
Green Manures are simple to apply, simply scatter seeds of plants such as Red Clover over the soil, lightly rake in and leave to grow. In the spring when your ready to plant chop this down and dig it into the soil 2 weeks prior to planting.....its a simple job. in early spring we use a green manure Phacelia a lot in the gardens, sown on bare ground and worked in a couple of months later, Mustard is another fast reward green manure, and can be worked in 6 weeks or so after sowing.
Cost wise, Green Manure Mustard will cost about £1.50 and cover around 20m2, this represents great value for the all the benefit it does...
Check out the Royal Horticultural Society website for more ins and outs of Green Manure application