Ant.H and Walter 63: thanks for your replies. I’ll incorporate the details in my final version of the story about my cousin. I am amazed at how crowded that radio/turret area was! I also got some information from Don Clark http://www.211squadron.org with a photograph of the installation in the Mark I. 

http://www.211squadron.org/blenheim_armament.html#VGO

When I was an ATC cadet in the 1950s I was lucky enough to get lots of trips in Lincolns at nearby Upwood. One of my favourite places, if I couldn’t get up front, was in the rear turret, which seemed quite spacious, but the Blenheim obviously was another kettle of fish. My admiration for my cousin goes up one more notch.

Laurence