Basil, OneDotLow,
Thanks. The date of the KT B757 flight from Ovda to LGW, by the way, was Monday 22nd March, 1993. How do I remember that? Don't ask.
But I must emphasize that what I saw on that B757 was very clearly a refraction phenomenon, related to a dramatic and highly-localised change of air density (presumably), rather than any hint of condensation. As I mentioned before, it looked almost like a transparent plastic washing line stretched between the tip the root of the wing which constantly moved chord-wise, forward and aft, but only a few inches.
I'd had one small bottle of red wine, maybe two at the most.