Hi
@Jon
thanks for that.
I watched a video that said as a rough rule of thumb to go with keywords that return less than a 1000 results (I think it also said something like Amzon will say "more than 1000 results found though") and as you point out, this tends to be a longer string of words (long tail key words?).
For anyone else who's doing this, the one thing I've noticed so far is you would be surprised by how popular some niches and how relatively unpopular some are, so I think it's certainly worth doing the keyword research, even if it is very boring!