Belay that -was nodding off or something!
Since I have a bolt near at hand this time I just took it down and yes, it all comes back: there is a collar that threads into the bolt head body and retains the pin and spring.
Bit of a b-----r to get out that collar; reminds me I had plans to bore out a screwdriver with a thick shank and slot the end to fit the collar - that or use a piece of DOM tube.
The bolt head will turn 180° as you remove it from the bolt body; even this one was a bit of a fiddle to get out the first time.