Some nitrogen may be lost--especially if the present mix contains few legumes--which can be replaced with an organic plant food such as blood meal or fish meal.

I may be wrong, but I thought that wild flower meadows did best if kept hungry. Grasses want the nitrogen etc, but the flowers do better without.
That's the whole point about "unimproved" grassland being a rich resource of wild flowers. Round here there are grasslands that are being grazed but not fed for years to try to reduce the level of fertility in the soil to improve conditions for wild species.

Yes, the soil is better left barren and not improved.
There is a perennial wildflower (tallish and yellow) that is excellent for 'weakening' the grass too but i CANNOT for the life of me remember its name now - but it has a strong taproot that is the only thing that will compete with the grass and WIN.
You then find the flowers are able to dominate the area.

This very subject was mentioned in one of the episodes of the Chelsea Flower Show last week, if i still have it on record i'll post the name of that yellow flower !

Hi Geoff. If the area is small enough I would pull out the weeds by hand and dispose of them before stripping off some topsoil. If it's too big an area, then maybe spraying and then stripping away a few/several inches of topsoil will do the trick (can't remember how much 'they' recommend you remove for wildflower meadows offhand I'm afraid).

You're thinking of yellow rattle, Louise. It's often used in wildflower seed mixes as it parasitises the grass.

That's it Annie, thanks :-)

You're welcome. It's infuriating when you can't remember the word isn't it! :)

Well, I mentioned the nitrogen because even wildflowers need some nitrogen--though not all that much. My experience in our exceptionally barren desert soil is that most wildflowers, without nitrogen added, are stunted--1/4 or less of normal height--short-lived, and few-flowered. Adding about 1 pound of actual nitrogen (10 lbs. of blood meal) per 2000 square feet restores normal growth and bloom. More encourages more grass, and more vegetative growth from the wildflowers, at the expense of bloom. I admit that nitrogen deficiency is unlikely in the UK, except on beach sand or chalk.