To say "one morning in May I flew to London", should I say
(a) Ein Morgen im Mai bin ich nach London geflogen.
(b) Einen Morgen ...
or other possibilities?
When you have a when-indication then it is neither "ein Morgen" nor "einen Morgen" but "eines Morgens" or "an einem Morgen". - Eines Morgens wachte Herr K. auf und fand, dass er ein käfer war. (Adapted from Kafka, The Metamorphosis).
By the way, the word Morgen has different meanings:
I would say this is 2. case (Genitiv)
So you have an indefinite article on "morning" because you don't name a specific date and a question like "Q: Wessen Morgen im Mai? A: Eines." That expression is than followed by a dependent clause.
I think it could also have something to do with temporal adverbs
You can find a similar example on Duden in the examples list.