Holly, in France wrote:
....anything at all that will out-compete brambles and nettles?! I have
battled with a bank next to a stream for four years, trying to dig them
out and keep them cut. Now there are mice near a waterfall (which I
don't want to poison - cat and barn owls), coypu coming and going, the
dog has joined in the digging trying to get at the mice and the whole
thing is becoming more and more of a mess! I've just cleaned it all up
again as well as I can. Trying to keep membrane/polythene in place would
be a nightmare, the bank is steep and the dog and various wildlife will
root it up. So...either I plant some very aggressive ground-cover or I
think I'm going to have to resort to Roundup for as long as necessary.
Any ideas please anyone? East facing bank, mostly sunny, never dries
out, floods half way up a couple of times a year.

Going at it, cutting them and digging them out will eventually kill
them off. It seems you've done this with no results. I am horrified to
read that you would result in using chemicals on the banks of a stream,
with owls, mice and other animals living near by. Why do you want to
get the nettles out? Doesn't this help the banks to stay put,
minimising erosion, contributing to the ecological habitat of all the
creetures around you?

Now, if you were to dig it all out to plant a salix say, perhaps that
would very much stop the brambles and nettles - clear all that site
once and for all and you would end up with a very nice area. But don't
use the chemicals please Holly )