Not that surprising I thought, presumably someone has activated the
dark satanic millthat is the UL, and is e'en now extending their
all seeing eyeover the blasted land of Cambridge.
But no, further investigation (by the means of cycling through Milton) revealed that this was in fact a very small cloud centred on the Jane Coston Cycle Bridge, and from that bridge I could see clear sky all around.
Very very strange. And wet.