Thanks for the reply Geriaviator, and the story.
I also got some information from Don Clark
www.211squadron.org with a photograph of the installation in the Mark I.
www.211squadron.org/blenheim_armament.html#VGO
I am amazed at how crowded that radio/turret area was! It turns out that there was only the one seat for the gunner, facing aft, and he had to reach round the gun to get at the unreliable radio at the back.
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