Re: "158 of which are Category 1" / severe illness / helmet
Vic
found this thread on PPRUNE which gives some details of the back seaters ejection seat - looks like they had to jettison the hatch with explosive bolts before ejecting
http://www.pprune.org/aviation-history-nostalgia/465563-canberra-bomber.html
as for WH876, that may be an oddball as it was used for ejection seat trials and may have a one-off fitment
as for my thought about the Nav not having an ejection seat, apparently that was the case in the B(I)8 - in the nose again