Thanks Melissa,
That's another way I hadn't thought of. I am so glad I posted the question.
I have just finished 'inserting' the blue trim. I cut out a blue frame 1 inch wide then folded in the edges (1/4"), cutting into the internal corners, pressed, basted, Roxanne glued it, then hand appliqued it down. I think it has worked well - it has the same bulk as if I had inserted the seam as a border -but the corners are smooth without the extra bulk of overlapping seams.
I haven't taken a photo yet - my house is full of workmen---I would like to have put a scalloped edge border treatment on this one but I don't think there is enough room - I have placed the petals too close to the edge, and I don't have enough background fabric left to add on a little and I think it might be too much blue if I used that to fit in a scalloped edge.
Anyway that is for the future. It might have been better to plan a bit more but I don't mind because it has been a real learning experience for me.
Once again thank you all for your suggestions.
Nan