As far as i read U 864 was running surfaced when performing the zig zag course, being observed by the english sub alright.
Only when the english sub had fired its 4 torpedos, U 964 dived to evade at greater depth, but ran into the fourth torpedo. So one can hardly speak of a submerged duel?
There is obviously a fault in some report
Also the U 864 carried the aforementioned parts and plans and mercury, but no uranium or such. This was tried in another boat bound for Japan, with the appropriate hull name of U-234.