Made some progress.
The Finn book is no use. Picture caption is "U-boat destroyed in Leigh Lights" is misleading at best. No source is given.
Attached is an extract from part of the UBAT contained in a staff assessment. After an accurate depth charge attack the Lib went back three times using its Leigh Light to sweep the surface to seek evidence of damage. On the second run it used flash photography and the mystery image comes from that. The description of a dark circle, a white centre and a white ring fits. The six beams remain unexplained but are nothing to do with rockets. Some quirk of the flares used for the flash photograph?
The date of the attack is confirmed in AIR15/138 as 24 September 1944. This comes from the form completed by the aircrew so must surely be correct. The squadron log seems to have a huge clerical error with the flight being missed out. Missing out a promising attack seems incredible.
The Admiralty assessment was ultimately "Probably sunk", classification B. The fact that Niestle gives no U-boat sunk on that date in that area might mean that it escaped probably with material damage.