Green manure advice

I am on my second year working this particular allotment and looking at my compost bins I will only have enough to spread over one bed at the end of the season. I thought about giving green manures a go but know nothing about them. I was looking for something to plant after I have lifted spuds, onions, sweetcorn etc. Anyone able to tell me:

A type of manure suitable for this that I can plant and leave over winter?

When to plant eg which month?

When to cut and dig in?

I am planning on doing a few beds so will be buying a fair bit - I have seen a few people (particularly DD) recommend Moles seeds. Anyone used their green manure?

Thanks Trillium!

Im sure they're helpful but I haven't found them anywhere near as beneficial as large amounts of horse pooh.

I am sure your correct Lardman but I do not have a trailer and have tended to find my car boot will only hold enough for to do one bed. Last time I did it the missus went mental because of the smell. Some people are just too fussy. I cleared up most of what came out of the bags when they fell over when I did a sharp left turn.

Im desperate for a cheap source of organic matter.

Pity you're not closer to me. I keep my horse on a yard that has huge quantities of rotted horse poo, they don't use weedkiller on the grazing fields and they don't buy in any hay. I also have 6 daleks and 2 big wooden compost bins, all full, and more garden waste/piles of dead leaves than I know what to do with But there must be other people like me who are close to you.

Im desperate for a cheap source of organic matter.

At home I dont have a problem getting stuff to compost. I have one neighbour that drops off his hedge cuttings, another who gives me his grass cuttings and I mow the grass/pick up the fallen leaves for the old lady nextdoor. I also have my own grass, hedge and kitchen waste so it means I always end the season with at least 4 full darleks. My problem is it is not easy transporting stuff to the allotment but my own garden does very well.