I see now,is land of severe disadvantage. 1st drain well, dig 1 metre x 1 metre trench from top to bottom down hill, down side of wall & also trench where water collect, around edge of field, possible also put in soakaway pipe, across middle of field & direct to side,downhill ,all 1 direction, so off !, this will keep rush & water plant down, eventually & land more balance. Spray all weed , thistle, nettle, & fertilise.Maybe plant tree down boggy area, many pine tree soak up water,or fruit tree for animal to eat also,when fall. This is all you can do. Nature always revert