[Casey76]

Since the chickens "went away" I re-seeded the pen with a mixture of grass, and a wild flower mix in a... I want to say sarrasin but that's not right, base.

Unfortunately, I also have a whole lot of nettles coming up all over the place too.

Can I use a selective weedkiller (specific for nettles, bindweed, brambles etc) on grass which is still in the cotyledon stage?

I can get the exat name of the chemical tonight, but bearing in mind I'm in France, so we may have different types of chemicals than those available in the UK

[New shoot]

The products we have in the UK have Fluroxypyr and Clopyralid in as a herbicide, but the only problem is that they kill all broadleaf plants, so will kill your wildflowers as well.

Can you carefully spray the nettles with a glyphosate based weedkiller 

[smud6ie]

The nettles should grow faster than the plants you don't want to kill so when they are higher use a weed wiper with glyphosate.

http://www.bfs.uk.com/productinfo.asp?v06c41q7p6=EFH&v06Qr5pT4172=EH
smud6ie

[fatcat1955]

I use Verdone on my grass but that will kill the wildflowers too.

[Alastair-I]

Nettles like a rich soil, probably the end-result of chicken-keeping and a steady accumulation of avian fertiliser.  A wildflower meadow is typically very nutrient poor, and has to be if the flowers are to compete with the grasses.  If the nettles are doing very well I think this is a bioindicator of future problems that you could have if you're going for a wildlfower meadow.

It might be worth sacrificing the wildflowers for the time-being.  Grow the grass hard and cut'n'remove for a couple of seasons to reduce the fertility of the soil.

[Goosegirl]

You could make a protective "spray-break" with plastic sheeting from compost bags or whatever so you can spray the nettles without it damaging near areas, but wildflowers do grow on poor, thin soil as Alastair-1 said.

[Casey76]

The weedkiller I have is triclopyr.  I'll be using it on the newly sprouting nettles on my next-year-to-be veggie patch, but the nettles in the wildflowers will be picked out by hand... if i can ever bring myself to stand on the tender new grass

[Alastair-I]

the nettles in the wildflowers will be picked out by hand... if i can ever bring myself to stand on the tender new grass

No need to be delicate, you're going to need to hammer the wildflower area heavily if you want the flowers to dominate and not grasses and nettles.  Softly, softly will not create the poor soil and low nutrient levels that they need.