For those who like closure, I bought a Smart Lunar R1. http://www.amazon.co.uk/s/ref=nb_sb_...rds=REAR+LIGHT It didn't fit the seat post despite the blurb suggesting it will. However, it also came with a bracket to fit it to the rear rack. I use Mini Ortliebs so can't fit the light there. Good news, it fits the seat rail (using a lot of the rubber matting it comes with) and is in a great position. It doesn't mess the fold and is easily seen. The light is small, flashy and bright. I still carry spare lights and usually hang one off my panniers, but its great to have the main tail light sorted.
So if you have an MU, a Smart Lunar R1 is an option.