Yes this Sgt Gilmour was a WOp rather than a Nav but I know these things can get distorted over the years so I thought it worth a mention.
Sgt Gilmour's Service Record would still be the best way to go to narrow it down.
I believe Bomber Loss Cards were only produced for aircraft that were lost outside of the UK - I'm not sure that an aircraft ditching in the channel would qualify if survivors were picked up?