Thank you for that, Michael. These unthinking assumptions often come around to bite one. As for the rest, I am diving back into the file to double check the fine detail - it really is a bit of a mess with lots of handwritten pieces.

What does seem certain is that Gerth wrote first to Suadicani directly on 1 September 1917; he can't have been at Boyardville for long so he was keen to get away. Suadicani replied on 5 January 1918 and the letter was delivered on 10 January to the French authorities; it seems likely that Gerth received it on 14 January after it had been decoded. Of course, there may have been more letters. Suadicani's letter said that a boat would cruise abeam of the Chassiron lighthouse during the nights of 12-18 January - so Gerth was cutting it fine. There is much more detail and instruction.

I'm wondering whether there weren't two u-boats involved: one waiting during 12-18 January; and one with Rudi (UC 50) planning to check during January other extraction sites for a back-up attempt in February.

One other page which may shed light: the files contain submarine contact reports for, probably, 5 January 1918 (so not UC 50). Ten miles from Chassiron, a French aircraft, one of a pair, threw a bomb by hand at a submarine (no bomb launcher); the submarine replied by firing a machine gun which hit the wing of one of the planes, and then 'dived quickly and disappeared'. The same afternoon about 1500, three French aircraft found 'the same' u-boat engaged in a fight with a British steamship. One plane attacked and 'fired a bomb straight on the bow of the submarine at ten metres'; the submarine dived and disappeared.

A further failed attempt at escape from Boyardville by u-boat in May-June 1918 involved Commander (possibly Captain) von Niessen and Captain von Dewall, 'former Commander of the fighting squadron attached to the supreme commander of the German Army'.

NB: Boyardville also contained the French school 'des defences sous-marines'.

I will post more after further investigations.

Best wishes
Chris