You will only succeed if you first get the fertility down. This is the opposite of the normal gardening habit of worrying about lack of fertiliser. Wildflowers evolved over millennia with no fertiliser; over-rich soil is not natural for them.
Since World War Two, landowners have poured artificial fertiliser on fields, creating un-natural excess. Aggressive plants, such as wide-leaved grasses and nettle, then outcompete normal wildflowers. To succeed with wildflowers you must get the “NPK” nutrient level down to pre-fertiliser levels.
Nitrates (the ‘N’) is easier than the other two, because surplus nitrates get destroyed over time; you then find clovers do well (because they make their own nitrates). But only when the phosphate and potassium (the ‘P’ and ‘K’) levels fall, can you achieve the magical results with wildflowers that you dream of.
Heavy rain can gradually leach thin/stony soils, but clay soil holds on to fertiliser.
Below the NPK ‘wildflower threshold’, nettles give up and the magic starts.
Nutrients are reduced by repeatedly removing cut plant material from the site, and ensuring fresh fertiliser contamination is avoided from adjacent sites.