Ah hah! G-AVTN in 1975.
It's documented on the glenforsa website (I wish I'd started there) and on aviation-safety.net but can't find anything on AAIB.
The riddle of the lost flight | This Britain | News | The Independent seems to have the whole story but who in their right minds would think a pilot seemed competent enough to land at Glenforsa at night. It gave me hiccoughs to try to land there in the middle of a sunny afternoon!
Edit: and more recently
http://news.bbc.co.uk/1/hi/scotland/3467055.stm
Even mentions the scallop diver!