Thanks for that Eamonn - you may have answered the pitch query too - it may be nothing more than being different.
It would be nice to stock them in many different sizes, and that is my ultimate aim, but cash flow prevents me from doing it all at once. Therefore I want to begin with the most useful sizes. Nothing is yet set in tablets of stone, and I'm extremely happy to hear opinions on size and other issues here, so thanks for that.