I would be tempted to rotavate now, G man, then you can feel that you have made a start - assuming that you are in one of those rare areas not covered in snow?
About the only green manure crop you could try at this time of year would be field (tick) beans. These would be good for the soil by producing nitrogen nodules on the roots.

Your other option if you're going away for six months is to cover the area in thick black sheet plastic, anchored down - this will kill off most of the weeds present, and you can then lift that and prepare the land when you return.

Yes, I agree with Bamboo. Put plastic over it. Otherwise the weeds will be really difficult to get rid of later, particularly if there are perennials such as dock or couch grass. Make sure you weight it down with pegs or bricks etc. The weeds will rot down in the dark and help feed the soil. It turns out lovely and clean when you lift it again.

That's a lot of plastic, though. Not sure just how environmentally friendly it would be.

Very unfriendly in terms of the environment! Also I question leaving ground covered like that for 6 months... yes it will kill the weeds (or at least some of them) but what is it going to do to the soil structure?

I had relatives who did this for a year in an old, walled garden when they first moved in - after that it was turned over and composts added to it, and its fine - as soon as light, water and air is allowed to enter, the soil starts to recover as the micro organisms reappear.

Well it is Gman's choice but not something I would do.

A full plastic cover as MG says is not good for the environment for such a large area. The cost of a porous membrane would exceed the cost of weedkilling and rotovating now and repeating the same treatment in 6 months time. It would take many large pegs to hold down, if it should blow away it would be a waste of time.

Thank you Dr B

Just to add a little more to the mix, a large sheet of plastic would form a cachment area forming a pond in wet weather and becoming static in dry weather, it would also be infested with many flies which would upset neighbours and if you make holes in it weeds will grow through. The soil structure underneath will dry out and it will take months to return to fertile soil.

Another consideration would be the possibilty of a gale during the six months that Gman is away. What would happen to the polythene if blew away and what damage to third parties is it likely to cause?

Another option is to leave it for 6 months, cut it two or three times, weed kill, rotovate and by the autumn you should have it cultivated.

Rotovating won't necessarily get rid of deep-rooted things like old dock and dandelion; leaving it turned over through the winter and spring will allow severe leaching of nutrients and destruction of many valuable soil organisms.

If you have the time and money I would cover the whole area in about 2 - 3 inches of compost and/or manure and then leave it alone.

Then when you return try to dig up as many roots as possible of deep-rooted things like dock and dandelion (of course that would be better before you go but is very time consuming and can't be done when the ground is frozen) - follow that by rotovation and you should have a good base for growing.