Yes I know what you mean! I always struggle with Merlin and Hobby without a good long look. I had a similar but different experience on last week’s ride. I was high in the Dales and looked down to glimpse what resembled the marsh harriers I see quite regularly In the coastal reed beds. But the habitat was wrong and it was clearly after curlew nests. Only then it twigged it was a hen harrier and a look in the book matched the mental picture that had lodged.One of those annoying fleeting glances of a hawk where you know its not the normal kestrel or sparrowhawk...but too quick to really gather anything of note.
Smallish, dove sized, Hobby or Merlin I thought, erring toeard a Merlin because of its darker appearance.