Robin Hogan and others have been putting together some very nice material looking at observations of the Ash cloud from satellite and ground based instruments, particularly those in Chilbolton, which you can see here (powerpoint presentation)
Particularly interesting are the apparent signatures of volcanic products at the ground. Robin will be updating the presentation as more analysis becomes available. Expect to see some of this analysis on Friday at WCD.
Also see some images of the ash collected at Lerwick on Thursday here showing the irregular shapes of the crystals (thanks to Chris Westbrook)
Giles Harrison and others have also been trying to measure the plume in-situ, early results available in a Met Office press release here