As thick as you can... when any leaves make it through pull as much root as you can and slowly it looses the will to live!

Thanks for that - yesterday it was going through my mind - in dread - that I would have to dig it all out anyway. I think i'll persevere with the mulching and can I just keep layering the card and mulch etc after the first crop? - presumably it gradually rots away and gets incorporated. So more mulch untill it gives up must be the right idea!
Louise
PS: I find I can't go pass a source of cardboard now without thinking can I stop and get it in the car!!