Thank you, Anders. Useful source. I'll dig my way through the book. I have also just found a mention in Tomas Termote's. Krieg unter Wasser - just recently also in English 'War Beneath the Waves', p. 88.

Termote confirms your point and say that the machine gun had to be hauled up each time the boat surfaced. It could be attached on top, in front of, or just behind the conning tower on a detachable pedestal. The gun was next to useless against aircraft as it could not be angled high enough. All of this would have made these ub-boats particularly vulnerable each time they surfaced and until they could get their gun into place.

Any more information from anyone, most welcome.

Best wishes
Chris