I've never seen anything like this before, at the end of a dissapating
contrail there was what can only be described as a black 'streak',
perfectly straight and very noticeable. The whole effect lasted a
minute at best.

A bit of Googling (unfortunately most of it's conspiracy theory
nonsense) suggests that this is the answer:
http://contrailscience.com/contrails...es-chemtrails/
I think the "edge shadow (volumetric shadow)" is what I saw,
the contrail was lined up with the sun.

However contrails must be crossing the sun all the time and I've
never seen that effect before. I wonder if you only see it if the
sun is at exactly the right altitude and the observer is in exactly
the right position wrt the sun & contrail.

Has anybody else seen this effect, it was definately a what the
hell is *that* moment!
--
Col

Bolton, Lancashire
160m asl