hello stuart, couch grass and every other known weed was in our negleted plot 3 years ago. start small with the cultivation. Buy as much polythene as you can or borrow it, weight it down well, and before you dig, once the weeds have gone pale and limp shear off the thatch that is on the surface and rake away as much as you can. Burn it so seeds cannot come back into cultivated areas. Then take a garden fork and lift each clod carefully, this helps to keep the roots whole, sift it through fork and with much aching of the back remove as much green/yellow/plant stuff and root as you can. Cover it over when youv'e finished digging and when you come back to it pull it through with a pronged cultivator tool adding compost or rotted manure as you go.
When you sow or plant hand pick out any rogue weeds and root and then keep on top of weeding by hand or hoeing every time you look at your plot.
After a year it gets easier and then you hardly see any nasties.
Keep unused areas covered until your ready to tackle them, that way you know weeds are being thwarted even though you hav'nt got to them yet.