very nicely written and thought out Jonathan. a focus group seems to be an appropriate way to get a groups' thoughts on this question, and food is a great way to get people to a focus group! I would say that 8 groups would be sufficient (though you technically would do them until thematic saturation, you should get some good ideas by 8). Re: the amount of data/number of groups, you can also consider your purpose - for publication you may want to this be more rigorous, whereas for QI, you might just get what you can get. I think using qualitative methods to identify barriers you DON'T already know about is a great approach.