You probably are aware of the fact but, just in case not, for the best wildflower meadow the ground really needs to be quite impoverished.
Have you thought about just strimming it over and removing all the cuttings, then covering the ground with coke-ash and cinders?
Yep, I am aware that wildflowers prefer poor soil. I wonder how poor though?
Prior to the 1st year, the ground had been covered with brambles for a long time. After I cleared it and dug it over, I planted the wildflower seed far too densely as far as the sowing guide on the tub was concerned. After some weeks and very little to show, I panicked a bit and watered it with a solution of Phostrogen or similar. (I was regularly watering it anyway)
Interestingly, that year was the best year - unbelievable mix of plants and flowers.
Subsequent years I've still sown the seed more densely than advised, and it's largely worked, but not been so good, culminating with last year which was the worst show ever.
My thoughts are :
The fertilizer actually helped, despite the "poor soil for wildflower" theory.
The self-seeded stuff overwhelmed the garden in subsequent years before the freshly sown seed had chance to get going
The fallow soil, being freshly dug, was preferred by the seed.
I redig it each year, so there aren't any perennials but it does mean that self seeded and freshly sown seed should get an equal go, since I'm sowing onto bare soil.
This year I think I'll water with a bit of phostrogen or growmore and see what happens, or see if I can find enough compost/manure to cover the ground before I dig it over.
Now having viewed the time-lapse movie, I was rather started by what I saw at around 3m 12s . . . a greater-checked sheddybum! 
Oops!
Ian
