I had given up hope of the nesting dippers, because I had not seen one for over a week or so, but yesterday lunch time, as I was watching a pair of grey wagtails flycatching from rocks in the stream, I thought saw one fly past.
I moved up to the stream, to where I could overlook the old pipe I had seen it fly into with the nesting material, and yes, it was there, but it saw me and took evasive action, flying down the stream, but was soon back, this time overflying the supposed nest site up to where the brook fall down the weir at the mouth of the tunnel. It paused and looked around.
I had moved into more cover, and it could no longer see me, so it flew back down the brook, closer - it fed briefly, flew to a rock nearest the hole in wall and then flew up into the pipe, instead of going in it looked around up and down stream, towards me, and then confident no risk was visible, it turned and ran into the pipe. I left.
The place where I watched it where many work colleagues come and sit and eat the lunch on fine days, hopefully it wont be too disturbed by them.