Ah, that's it - I'd forgotten his name.
To be honest, it wasn't quite as terrifying as I'd expected becuase somebody had installed some wire zip-lines which we could put a pully on to haul kit (and ourselves) over the pools.
That said, I agree that if one of the bridges suffered a big collapse, then you could easily be dragged under water still attached to the bridge. Need I say more....it would be less than pleasant....and also single 8mm bolts in slate roofs aren't all that confidence inspiring anyway.
The worst thing was the cold - a fair old wind whistles through those mines.
I wouldn't encourage anybody who isn't pretty competent to have a go at it because you could come a major cropper if you went off route or something collapsed, but it's a fascinating trip and has a real "Mines of Moria" feel to it.