The only other option seemed to be to build an arch, but this is anything but easy even with a former* (which we didn't have) and an abundance of suitable stone (we had plenty of stone, but you really need lots of regular wedge-shaped pieces - not common in the greywacke we were building with). So, much to Mike's regret, we gave up on the bridge idea and just continued where we'd left of on the wall.
However, all was not lost. As we were taking down the next section of collapsed wall, we discovered three huge flat stones (the largest over a yard long). So, despite the doubts of some as to whether it would work, a few of us set to work to bridge the ditch with them, leaving the 'doubters' to continue with the wall.
* A wooden arch to build over, which is removed once the stone arch is in place.