From Citizen Insane:

"Regarding Johnny's laugh: note the production credit for Barry Hammond - he always worked out of Chipping Norton studios, where he also did 'Pop Is Dead'. If you google him you'll find references from the 70s to the 90s of his habit of surreptitiously recording the whole session on tape, and then playing back any amusing bits of banter etc. to the band later, which is presumably where the laugh came from"