Google Phlox paniculata. That is the herbaceous type which you may well have seen. Daft to call them American Phlox as it is a purely American genus, ranging in size from inch high Alpine types to the tall border ones.

Thanks again for your answer Owdboggy, I have looked up paniculata and you were absolutely right so I'm off to the nurseries to get a few

There are some beauties of them. I especially like the ones with a white eye. They are a bit prone to mildew so need a dampish soil or plenty of watering and some fungicide, but worth the effort.