Sara, here in N. America we often have to do something about the weed seed bank in the soil. Typically we'll "bomb off", ie spray with glyphosate to kill off the weeds. Then we let the site sit for a couple months and allow more weed seeds to sprout. Then we bomb it off again and do a dormant seeding in late fall. It probably gets much colder here than where you are, so we have the ability to do a "frost seeding" whereby the seeds are placed in late fall after all chance of their sprouting has passed. Then, the freeze/thaw cycles of the winter do an excellent job of placing and stratifying this seed. Not sure how much of this is applicable to your situation.

Sarah, Tom is right that you need to get rid of the coarse, commoner weed species which will grow once you have cleared the area. Also you need to bear in mind that our choicest wild perennial species grow on fairly impoverished soils so do not add any fertilisers, compost etc. And you need to realise that growing a meadow is not a one off project. It will need mowing annually in order to keep the flowering species going and to reduce the coarse grasses. You will need to remove the mowings, not leave them in place (to reproduce the effect of taking a hay crop). Another point, don't buy 'wild flower mixes' from just anywhere - many contain species which aren't even native to the British Isles. This site will help you get started. There are lots of other sites devoted to wildflower meadows in the UK if you just Google. Here on GW most posters are in the US, so their conditions will be different from yours.

Christopher Lloyd did a glossy (and verging on a coffee table) book... but helpful and not overly puritannical or dogmatic about species choice. It is a project which can stretch over several seasons until the seed resevoir in the soil is diminished (you can only do the chemical death route once...although it remains the best way if you can stiffen your sinews and get out the glyphosate).

Cotswold Seeds, Emosgate seeds, and various online sources (British Wildflowers in Norfolk) have been my choices for native seeds