Obelixxk, pauper, Steve and Fairy - Yes, cardboard is good, but I would have to find lots and worse than that, lots of rocks and bricks to hold it down. I don't have a car to collect all the stuff so I think it is going to have to be dig and leave. Thanks for all your ideas though - wonderful people.
You can buy metal U-shaped prongs used for holding down weed suppressing fabric and use those to hold down your cardboard. My guru gets hers from shops that have deliveries in large packing - household shops and supermarkets. They're happy for her to take it away rather than have to recycle it themselves.
Why are the simplest ideas so easy to forget? I pinned my membrane down with pegs. That would surely work for cardboard. Even wee bits of stick.You're not wasting all that digging Forester. Folk on here will pound you into submission with good ideas. They're brilliant!!
My friend has an allottment so that she can feed her children, her and her husband are on very low incomes. She gets someone in to rotovate the allottment in spring and says it is very inexpensive. The allottment holders get a special rate.