More on the cascade of lights
Thanks to Bella for the suggestion that the cascade of lights may have been a meteor shower. She rightly points out that there has been activity around this time and, sure enough, even a superficial search reveals that around 12 August was the expected peak time for Perseids.
What I saw on 10 August was rather less dramatic, gentler and more sparse than any of the spectacular pictures to be seen, for example, on Google Images under either Perseids of Meteor Shower. On the other hand, none of those images shows the tunnel shape and the relative stillness I found so intriguing on the Northumbrian sky. But these are probably minor divergences compared with the very satisfying fact that a plausible explanation has been found.