Thanks everyone for the comments.
Declan, it was quite a scary sky. The clouds were very dark and stormy in appearance for a time and on the motorway they seemed to almost touch the ground in the distance which was why I was watching for funnels etc.
Brian, I'm pretty sure it was the mountains indeed. Trophographic lifting and all that. I suspected a hint of NLC formation before midnight but the suspect vanished when the Sun dived lower.
* I'm sure I spelt trop wrong but it's too late at night for my brain to work
