Reply to simpson-allen
Hi there, I wouldn't advise just building raised beds on top of your soil. If you use plastic to prevent weeds coming through, the beds will become waterlogged, as you say, but the plastic will also prevent worms entering the beds, which are vital for mixing in organic matter, bringing nutrients to the soil etc.
I would grow a large patch of potatoes in your weed-root-infested soil. They will help to break up the soil (improving drainage) and prevent the growth of the weed roots, allowing you to concentrate on digging a smaller area. And I'm sorry to say that the best thing you can do for your soil and your vegetables is to just dig a small patch at a time and remove the weed roots. It's quite satisfying!
Do use your soil...if your plot hasn't been touched for 18 months then it's likely to be organic, full of earthworms and plenty of organic matter and nutrients. It's also likely to have a high clay content if its waterlogged, which means it will be even more packed with nutrients (digging it and adding organic matter will improve drainage).
Good luck and keep at it, the results are well worth it.