If the hedge is hers but grows over onto your land you are allowed to abait the tresspass up to the boundry.
If, for arguments sake the hedge is 4 feet thick and the centre of the hedge is the boundry then you can cut back 2 foot all the way to the top.
There is little you can do about her side of the hedgeor the height on her side. You could cut right back to the boundry and then errect a fence tight up against it though.
As the land registry will tell you measurements on deeds are not 100% accurate so be careful if you do have any work done.
Take lots of before, dureing and after photos or even film the work being done.