MReyn24050 has it.
XA390 had the hold back break early on cat launch. Ditched 05/07/57 over the bow of Bulwark while she was anchored in Bangor Bay.
Crew picked up OK and aircraft recovered by HMS Barringtion a week later.
321/B was ditched after engine failure 25/09/57 so it may have been this aircraft that was seen in August.
Regards
Ross