The canopy 'clearance' was required for all marks of Lightning due to the heavy metal reinforcement in the top. There was a sad fatal (1966 I think) from Wattisham where an ejection failed due to a stuck canopy, and in the subsequent !FORCED LANDING! the seat ejected from the INTACT aircraft when the canopy was released by a bump in the field in which it had crashed. A valiant effort which resulted in the death of the pilot. One puzzle here (to sidetrack this sad thread) is why the ejection drills were not changed until 1973.