Guildford, Surrey Extreme frost 12/12/12

Neidpath viaduct crossing the Tweed by Peebles. Also known as the 'Queen's Viaduct' it carried the Peebles to Symington Caledonian Railway where it once connected to the now West Coast Mainline. 

The description fits, and it was just ahead of the cold front so was the precursor to the current cold spell. They not something I have seen before. Thanks for the info.

Morning Mike, they look like Nacreous clouds potentially:
What is so special about Nacreous Clouds- mother of pearl Polar Stratospheric Clouds
WWW.NETWEATHER.TV Way up in the Stratosphere there is very cold air above us. It's causing excitement for those who watch such things as a precursor to a possible cold, winter spell. This air can produce Nacreous Clouds PSCs  

Yeah, it's the Alabama Tornado, rated around EF3/4.