I've just chatted with my pontoon neighbour, who's a Port of Bristol pilot. His info is that the barge was late arriving, so there wasn't time for the new gates to be fitted in their final position. (Rescheduled for Spring 09)
The Mersey Mammoth had to leave yesterday for another job, but with a square bow and max 6 kn it probably couldn't make way against the strong wind and tide - you probably saw it turning back to anchor.