Just read this thread...I would expect that the Rylands principle wouldn't have application in a situation where the object was neither stored on the land (If this is a one off spray) or, if the shed was sprayed by a 3rd party, brought onto the land by the 3rd party.
It is likely though, that you do have a claim in tort, a civil action, for the damage caused. As noted previously by Ms. Gold, you should get all the relevant information. (Making sure that your car was parked legally). Then you should speak to a solicitor; I would think it likely you have a claim. Hope this helps