One thing about the punt trip that I didn't mention was that there were loads of gorgeous dragonflies and damselflies on the river. Googling suggests that the majority that we saw were probably Calopteryx virgo (and C. splendens as well). A pair of them decided that Mark's hat was a good place to rest for a couple of minutes while copulating. Another one came and sat on my knee for a while.
This also suggests that the upper Cam at least is nowhere near as polluted as it is popularly reputed to be.