As said above, XR 721 crashed on 5 Jan 1966 with sadly fatal results, I quote from the IAM report:-
'The cockpit canopy jettison system had fired when the pilot initiated ejection but one lock or shoot bolt had failed so that the canopy could not lift from the front and it had remained held by that shoot bolt. The ground impact was sufficient to shake the canopy off and thus remove the interdictor.'
The report then says that the pilot would have had to pull the handle again to set the seat off or possibly the seat was forced up by the presumably very rough landing to start the sequence again. Whatever, a very sad ending after the almost impossibilty of deadsticking a Lightning.