Hi there novice3,
I agree with nutcutlet on the yellow rattle for your area of grass – I might even suggest you see this as a 2yr project where you only sow/plant plugs of yellow rattle in year 1.
I’m trying yellow rattle on a much smaller scale in a semi cleared, area. My grass mound was sown with wildflowers in mind too but the young grass at just 2yrs still choked space for the flowers to grow. My sowing was last November so I've yet to see this plant growing.
I found this info for you:
“Can Yellow Rattle be used to reduce grass growth?
Yellow Rattle (Rhinanthus minor) is a hemiparasite which photosynthesizes for itself but also parasitizes the roots of some plants, especially grasses. Because it removes nutrients from the grasses, they produce less growth and the balance of competition is changed in favour of the wildflower species in a mixture. Yellow Rattle can also build up in patches and then die out, especially if it the host grasses die out. The bare patches are then open for colonization by wildflowers.”
Source: http://www.scotiaseeds.co.uk/documents/FAQs.pdf (more on wildflower meadows there too)