Is Spanish Flag an iris? I would just leave the seeds on the plant for as long as possible, Rosaline, and then collect and sow them. As you are cocerned that they are not very developed then they migh not be viable - but there is no harm in trying.

She may be referring to Ipomea lobata Bulboholic.
If so, yes you can. When the seed pods go "crispy" they are ready to harvest Rosaline. The seeds inside will be black, about 3 or 4 mm in length.Leave them on kitchen roll in a warm room for a few days to make sure that they're dry (as all ipomea seeds are prone to mildew if not dried) and then store until early April when you'll sow them.

Thanks it is an Ipomea but sightly different ...Very helpful.

That's the one then, Ipomea lobata. I grew it a couple of times at my old house but it didn't do too well. Must remember to try it here next year.