I would recommend Phacelia, as it grows very rapidly in summer, deters weeds, and is easy to cut down and incorporate into the soil. At the same time, you have the bonus of beautiful flowers which attract dozens of pollinating insects. I'm not sure where you can find see in the UK but here in France we are lucky that you can buy 500g boxes of seed for around 9 euros (about £8).

Agree with Bertie Phacelia is great for this. Most GC's carry green manures these days, otherwise you can order on line from Garden Organic or most of the reputable nurseries.

Thanks Bertiefox and Moon growe, I will have a look on the internet to see what I can find.