These photos were taken back in September - I couldn't believe it when I checked. This was in the early stages of the works. I haven't actually officially found out what they were doing, but from observation it appears that they have been replacing timbers in the structure (does it have a name?) that guides boats through the swing bridge when it's open.
It's been a long process, and much of it, it seems, could only be done at low tide, with all the equipment and the barge/pontoon being moved downriver whenever work stopped, enabling the bridge to continue functioning throughout.