Three useful sources:
1)
http://www.naval-history.net, the reference site on Royal Navy history online, two useful pages:
http://www.naval-history.net/xDKWarDiaries-Contents.htm, for various details (but none covered your date)
http://www.naval-history.net/xDKWW2-3900Intro.htm, for a very detailled chronology of the war at sea (this chronology is not always easy to read, as often the events will be listed on the date of the departure of the force/convoy rather when they took place some days later).
On 30 August there is written (among many other events):
"Norwegian steamer MARSTENEN (1832grt), in convoy WN.11, was sunk by German bombing in 58‑23N, 03‑37W, off Duncansby Head. The entire crew was rescued by destroyer CATTISTOCK. The crew was later transferred to examination vessel MINNA. "
Duncansby Head is not far from Pentland Firth.
2) fold3 has online the British Admiralty War Diaries.
Browse WWII -> WWII War Diaries -> A -> Admiraly War Diaries, then one file per month or sometimes per two months periods.
In the file for 8-9/40, 30 August starts page 630 and this page confirms the above information, giving some more details (the attack was made after dark, the steamer is reported sunk by torpedo, etc...)
http://www.fold3.com/image/1/302053101/
Page 636 (
http://www.fold3.com/image/1/302053101/) also describes the attack, and maybe others (I stopped there
)
3) and as we speak of a Norwegian ship, the excellent website warsailors.com is very useful, there is a page on this ship (as on any Norwegian ship sunk during the war AFAIK):
http://www.warsailors.com/singleships/marstenen.html
More details are available here.
By the way, this last website has a very active forum, comparable to TOCH on quality and activity (I mean high standard
), but about maritime matters.
Hope it helps