I've had the chance to study a few more trails above SW London today and, whilst several of them exhibited distortion that might be considered wave-like, it was relatively random, short-lived and accompanied by a general diffucion of the trail itself. As such it is easy to see how these could have been caused by wake vortex or air current effects.
However none of these observations, or indeed those proposed by others above, coincide with the essential characteristics of what I and others observed on Saturday, namely:
* a single trail that remained extremely thin and well defined prior to evaporation
* a regular sine pattern that remained stable and consistent behind the aircraft for perhaps 20-30 miles
* a consistent wavelength and amplitude of the sinusoidal curve
* a pattern that was not replicated by any of the many other trails in the same area and at the same time that day, which generally decayed through slow and stable diffusion prior to evaporation.
I don't think we yet have the answer!