Thanks PK - have a lot of jobs on our hands.
Seeing as you haven't said organic yourself...
Take the plants you want to keep out. Cut what's left , rake it off, burn it when dry to give you potash.
When you have 3 inches of regrowth give it two doses of max strength " Round up " five days apart , the plants are very vulnerable to it at ths stage .
leave it three weeks then dig the plot .
Couch grass can split hair line cracks in concrete to wreck the concrete in a few years , It can go down 2 mtrs or more , travel like bamboo underground for a long way before it re emerges .
If you are lucky you can try mulching with a five or more inch thick layer every five months in the main growing season with a very light water retentive airy mulch in the hope that the couch comes up & grows in them mulch .
Giving the mulch regular feeds of liquid feed helps it do this . Then when it's poking through about 2 inches tall you carefully pull out all the greenery & runners , as soon as you see new growth of couch pull it out.
Keep doing that for five of six years & you just might weaken it enough to kill it off .