Thanks. I was in a window seat, 4 rows behind the trailing-edge emergency exit row. It probably was a de-rated departure. Apart from the vibration, everything else looked / felt normal. (It was a A-340-300).
The passenger sitting next to me (who, it turned out, travelled the same route regularly), spontaneously remarked, "that was scary". It wasn't quite worth a call to the Daily Mail but it was definitely unusual!