We find that the biggest issue for them, is predation of eggs / chicks / nesting birds.
Biggest culprits being birds of prey, corvids, and of course the fox. Brock also plays hell with them (as they do with all ground nesting birds) but obviously with the current legal situation, its a toughie dealing with them.
Other than that, good natural un-cut head lands, and breeding / cover crop strips really help, as do 'beatle banks'.
Something magic about spotting a hen with her brood out scratching on a summers day.
Someone once told me that to get an established population you need three things in large amounts, time, money and luck.
Fingers crossed for you!
And if you have a surplus of mink. . . . . . .give me a bell