The Hemel explosion 1.5 hours later, still from Harpenden. (See also this photo of the cloud of smoke taken after 20 mins.) It's hard to convey how huge this cloud is - the camera is set to a pretty wide angle, and the depot is several (I reckon about 4.5) miles away. All that you see is smoke. The rest of the sky was clear.
Fortunately, although some closer homes experienced structural damage, there weren't many homes very close, and there are thankfully few casualties reported.
Story on BBC News.
POSTSCRIPT: It was amazing to see the pictorial news of this event spreading via Flickr. This (and particularly) my other photos taken from 6.20am were some of the earliest taken and uploaded - well before the national TV had clear information - and as other photos followed throughout the day, with descriptions from eye witnesses, it was fascinating to follow the way people began to learn what had happened.