DeepC
There is much analysis and disussion of this event here:
http://www.ukweatherworld.co.uk/foru...iew.asp?fid=28
See especially the thread entitled "Synoptic analysis of mid-level storms....overnight April 30th/May 1st". There are many maps and radar plots of the event and if there is any information you would like to see I'm sure the members will share it with you. They are a friendly bunch unlike some of the prima donnas on this site!
I actually witnessed the early stages of the storm you saw as it developed over Dorset from my vantage point in Teignmouth, Devon at around 0030BST. My parents said it was one of the worst night-time storms they have ever witnessed as it came through Leicester at around 0330BST
Incidentally I think they have changed the site with the lightning plots - I've just noticed it only goes back a couple of days now.