Thanks for replying Tiny Clanger. That sounds like it could well have been it then. Usually the trails I see gradually become wonky as they disperse, so I dismissed the idea. Amazing, I have lived here for almost 20 years and never seen anything like it before. As you say the atmosphere must be unusually slow. I will be out looking tomorrow. Wish I had a telescope!
another thought though, usually planes contrails stay in pretty much the same place in relation to the ground, but this was moving away from me at some speed, and from a big line to invisible in the distance could not have been more than 20 mins.