Posted: 12/02/2013 at 17:33
You've had lots of good advice here and I can only give my experience. I got my plot a few years ago. Heavy clay, not much previous work done and waist high in weeds. I cut all the weeds down to the soil and covered as much as I could with heavy blue tarpaulin / plastic sheeting from a swedish company well known for selling furniture.... Meter by meter I worked that plot and took out by hand as many of the weeds that I could. It will take time but as has been said, using a rotovater only breaks up the roots, that is supposing you could get it through the mud.
It won't be easy but if you take it bit by bit then you'll get there eventually. There is'nt any point trying to work wet clay. I don't know where you live so don't know if you're likely to get much frost. I'd be inclined to cover what I can, it'll help dry out the clay and the lack of light should help the weeds die down.
As has been said, you'll need to add compost or / and manure to the beds to help make it more suitable for growing but there's still time for you to prepare a few beds for planting this summer. Don't forget that potatoes are good for breaking up the soil too, I grow them most years. Best of luck.