what advice you would dispense regarding the best meadow or plants to use in our typical Essex, wet clay in winter, rock hard clay in summer, fields?

my mother-in-law has a large field in Hullbridge that I would rather have as a wild space than a lawn. Currently it is a mixture of mowed lawn, overgrown lawn and weeds The ground has been fallow (bar the brambles I have now removed) for over 25 years without any pesticides so it is ideal for wild flowers or the like. Any suggestions would be appreciated.

Roto til and sow early autumn?
plug plants and weed out the brambles/ thistles?
Heavy scarify and top dress with seed mix?

In a damp environment,  a meadow will almost surely develop on its own. I have areas where I do absolutely nothing except for trimming out undesired specimens.

Try doing nothing for a couple seasons. A wildflower mix might help.

Tim Wells: It seems to me that the same plants that are weeds in a lawn are wildflowers in a meadow. I think that the main difference between a meadow and a lawn is how they are cut. Lawns are cut regularly to keep them short. Meadows are typically burned or mowed once a year in the late fall or early spring. I second Dale's suggestion of letting the meadow develop on it's own perhaps with the addition of a wildflower seed mix. I really like species that are already growing in the area: They are already locally adapted to the climate, soils, bugs, animals, and other plants. If it were me, I'd visit the local wildlands, roadsides, vacant lots, and parks and collect seeds, plants, or roots to add to the field. Collecting throughout the year will bring in a blend of spring, summer, and fall flowers.

Joseph Lofthouse wrote:I really like species that are already growing in the area: They are already locally adapted to the climate, soils, bugs, animals, and other plants. If it were me, I'd visit the local wildlands, roadsides, vacant lots, and parks and collect seeds, plants, or roots to add to the field. Collecting throughout the year will bring in a blend of spring, summer, and fall flowers.

I have to second this. I can appreciate the big "show" flowers my neighbors plant but I find a field of beautiful flowers is only a roadside away and costs nothing more than a couple minutes of clipping. The only role you need to play is of artificial selection by chopping-n-dropping the flowers you don't want and spreading the seeds of local flowers you do want. If you're feeling lazy, you can buy a bunch of seeds of the flowers you like best and they'll probably grow unless your land has extreme conditions. (Bonus points for medicinal or culinary functions in my book.)

Check out this thread that has quite a few quality posts on soil solarization:
http://www.permies.com/t/43714/soil/info-Soil-solarization