OK, so several of us noticed the same unusual trail yesterday but no one has come up with an explanation. I'd say that there are only two explanations:
EITHER it was a normal straight trail that was immediately distorted into the sinewave shape by air currents,
OR it was a trail made by an aircraft at high level that was flying a continuous sinusoidal course.
The first of these seems unlikely, since the trail itself retained its thin coherent shape until it evaporated in the normal way. Incidentally its thinness would seem to rule out multi-engine aircraft such as the B52 previously mentioned; it had to be either a single or very closely coupled twin to make such a very precise single trail. It is also difficult to imagine an air current scenario that would distort a trail so rapidly and precisely into that sinusoidal shape, yet leave all other trails nearby completely unaffected.
So, if the second explanation is the more feasible, what reason could there be for an aircraft to fly such a strange course for so long? I'm wondering whether. for instance, a reconnaisance aircraft with side-facing sensors would need to deploy alternating banking in order to direct those sensors towards the ground. Or could it simply be some sort of control test? Either way, I can't help feeling it must have been a rather uncomfortable manoeuvre to undertake for any length of time.
Any test pilots out there able to comment?