Hi Alan,
welcome to Germany and good luck with your project. I have a big vegetable garden near Cologne, I am using no dig and follow Charles advice in this website and have some of his books also. Regarding the polythene you can get good quality of “Silofolie” usually in the Raiffeisenmarkt (supplies for farms and gardens). They cut also to the size you need. The polythene sold sometimes in “Do it yourself” markets is not opaque and lets some light pass so the weeds and perennials will not die off.
The good quality polythene is covered also from the inside.
I checked for you it seems the nearest Raiffeisenmarkt is :
http://www.rwz.de/rwz-hillesheim/
Good luck
Udo