Green manures

This is worth a read PDF on green manure. It really depending on how long the space is going to be empty , what type of soil you have and how much you like digging  I thought about putting some fenugreek yesterday.

For short term to be dug in by the Autumn or before phacelia is probably the easiest to manage and the flowers are a bee magnet.. Mustard is fine but remember it is a brassica.

If you are talking longer term, to be left until next Spring before digging, then clover is a good choice. Crimson clover is probably best but red is also good.

I personally use quick green manures alongside cow manure. My favourite is a legume, Winter tares. It doesn't have the brassica stigma, behaves itself, is easy to cut and dig in, and supplies plenty of matter and plenty of nitrogen if planted at the right time. I will be planting up again late Summer to help next year's brassicas. Last year's Tares has done wonders so far for my cabbages.
What I would say is tread carefully with the rye grasses. They can get awfully big and very hard to dig in. Might almost feel like you're back to square one on a new plot.

just been to buy green manure, i bought phacelia. cant plant whole plot this year due to ill health. as not planted spuds this year will have lots of spare plot. like the idea as been mentioned about letting it die back.

Why not grow something like peas, they will improve the soil and might give you a crop at the same time. I tend to use broad beans, peas and french beans as crops to cover the soil especially at this time whilst there's still time for them to gro and crop. You can still dig them in if you don't want the crop.