Question
How can I effectively get rid of brambles,nettles, and rushes that are smothering an old stone ditch? Also a neighbouring site is overwhelmed by the above, and no doubt seeding into mine. How often should that site be cut to contain the problem? Thanks

Answer
Hi Shefra, If there are no desirables in the ditch, spray the entire area with glyphosate.  Sold under the trade name of Round-Up and available much cheaper as a generic, it's safe to use around potable water, does nothing to the soil so you don't have to be concerned about runoff and it kills just about anything you spray it on.  The more green tissue you can spray, the more effective it is.  Some tenacious plants may require a second spraying after they have re-sprouted.
Cutting your neighbors plants probably won't prevent them from seeding some, but seed could blow in from anywhere, not just next door.  Whenever you get a little re-growth, spray it again.  
If there are desirable plants, mix up glyphosate in a bucket, use a sponge mop and wipe what you want to kill.
Jim