From many years of experience: If you have bramble that's mature enough to do what you describe, anything you can effectively spray it with is gonna wipe out all that is near it - and you'd need a license to buy such stuff..................You're gonna have to go at it by hand, gauntlets, billhook and slasher, and grub the lot out ...it's the only way, and it isn't all done in one year, either!
Your neighbours should be happy to do their bit at the same time as you do it, or if not then they are morally obliged to allow you access to get the job done, I'm afraid it's one of those jobs that has to be faced head-on.
Sorry if this is not what you wanted to hear, but it actually isn't as bad as it seems once you decide on it, set aside the time, and then "Git-R-Dun"!....................atb mac