Today I measured the height of the bridge. The height is now marked on the rope that I used lower my digicam down to record sightings from the 'underground'. I put the camera on REC and gently lowered it down. The view was interesting enough, with the bridge pylons, huge beds of round stones and half-frozen water - a totally different world to that above the bridge. I could see no-one camping there though, which is of course great. I might have to lower my expectations on finding a new human layer here however.
Later I learn that the school has no material to sell - I will likely have to go with whatever I can find from the Recycling centre tomorrow. As we can only work until 4PM, I'll have do all my work there on Thursday.