Interesting story. U boat sources have nothing regarding the action suggested. I am sure something like that would have been well known. The lancs fitted with .5 turrets would have been with 1 and then 5 Group. Cannot discount that 6 Group put them into theirs of course. Uboats would unlikely have been on the surface in daytime, as it was far too hazardous at that stage of the war. Aircraft shot down by Uboats were usually from low level and whilst carrying out airborne attacks. To come across a U-boat on the surface would have been pure chance. Many Coastal Command crews went hundreds of hours and whole tours without sighting a single Uboat. If true, remarkable story nonetheless.