The idea is that the bell and its clapper beneath are set at a height such that the movement of the waters at high tide rings the bell, and that only the highest tide at that. As the waters rise due to global warming this would occur more and more often and not just at the highest tide, a warning to us all.
Alas the date and time chosen for the ceremony, namely mid-day on the 19th of September 2010, was no where near the highest tide you can get on the Thames. Indeed a check (on the iThing) of the PLA's tide table quickly showed that though high water was 10:54 GMT i.e. six minutes to mid-day BST it was merely 5.8 m at Tower Bridge, unlike the spring highs of 7.5 m.
However apparently the clapper can be "tuned" by increasing its length and hence reducing its height above chart datum. But another check of the PLA's site, this time the live tables, showed that the river was about 8 cm below predicted levels, most likely due to the weather system over the North Sea.
Hence we were to miss the bell ringing of its own accord, and so the artist, Marcus Vergette, was forced to ring it by hand. But there was free champagne on tap even for random spectators such as myself and everyone seemed to enjoy the moment.
Indeed please do watch, as it was the wait to have time to upload this video that meant this post is two weeks late!