How about Lunaria rediviva (perennial honesty), white flowers for several weeks, then pretty silvery seed pods which last through winter (mine still has its seed pods even after all the strong winds).

Gypsophilia paniculata 'Bristol Fairy' is probably about the airiest, most blossom like perennial. You might also like Limonium latifolia (Sea Lavender). The leaves make a clump at the bottom and aren't stunningly attractive to be honest - but it comes in pinks and lilacs.

Thanks very much Jaykaty and Bamboo. I do have a white Honesty (with variegated foliage), so I probably need to make it more widespread.

I will look up the Gypsophilia and Limonium . . . lovely!

Some of the mid-sized campanulas? Also the half-hardy perennial Nemesias and Diascias are somewhere between frothy and swishy, and can flower for months.

Thanks Teadrinker - we think alike! I've just ordered a few campanulas. I love Nemesia too, and have a few, but mine only grow to about 9".

Rats, my book reckoned 16" for Nemesia denticulata, and various Diascias at 1".

It's a puzzle Teadrinker . . . on another site Nemesia are described as 'mat forming' and yet max. height is given as 0.5 m! But they certainly are 'frothy' so maybe I could just put them in a tall pot?!

Try Ammi majus. This is an annual that will self seed. It has frothy umbelliferous flowers for most of the summer.

Thanks Volunteer - good idea. I was given some Ammi majus a couple of years ago, and grew it next to my Cotinus to give it support as it is so tall; must get some again (I should have saved the seed!)

Sheilabub I was given a very pretty light & airy Geranium Pyrenaicum 'Summer Snow' last year and liked it so much that I've just ordered seeds from plant world seeds - it is long flowering & should self seed - quite unlike the usual geranium so thought it might be of interest. Achillea ptarmica common name Sneezewort looks similar. I love both - so google & see what you think.
Spritz recommended (perhaps last year) Geranium Pyrenaicum 'Bill Wallis' which looks deep lavender so ordered those seeds as well.
Linmar

Wow Linmar . . . thank you. I think I might still have the Plant world catalogue, so will have a look (or Google your suggestions). The G. sounds great, and I remember Spritz's Bill Wallis . . . lovely. Growing purple flowers and white flowers is my main plan for this year, so your suggestions are very useful, and much appreciated :)