We've now watched the Reedham episode, and most enlightening it was! So it's not just a button the operator has to press after all....
What did amuse me though, was that a few months back there were bridge closures for maintenance. When walking under the bridge, a strong smell of paint was in the air. As the outside of bridge is still as rusty as ever, we did wonder what had been painted. From the tv program, it's obvious they have painted the bit on the inside, that was vivible as Michael Portillo walked over it!!