People who went to see this bird were only getting really poor, mainly obscured, views of the bird's head and bill while I was there: it looked nothing like a Grey Heron, the bill being heavy and bright orange. The bird seemed to be very wet which did not help. It showed briefly as it flew further away and I noted the upperwing as 'leaden'.
Having read up a bit it does not look much like a Purple Heron either though. Someone has already labelled it as a 'manky Grey' which is the most likely I suppose. Where it gets that bill from is anyone's guess though.