Spectrum Effect in the Sky - Any Suggestions

I was busy in the garden most of the day but did go down to Torness to do some long exposure photography in the afternoon.  

The blip is of a spectrum effect which I spotted just as I was leaving Torness - hence the rather unpleasant railings in the foreground.  I don't think it was a rainbow but am intrigued to know more.   Any takers out there.