what are the best perennials to plant on top of a windy bank ?

Hi Billybob,
Start with Primroses. Bergenia Cordifolia.
Nepeta Racemosa.
Coreopsis Majalis.
Astilbe Bronce Elgans.
Campanula Punctata.
Aster Novi- Belgii. (Michaelmas Daisy).Jenny & Lady in Blue.
Sedum Herbstfreude.
That should start you off.
Good Luck.

Is the area dry or moist and is it in sun or shade? These questions make a difference to what you can grow there, but here are a few plants to look at. Obviously there are more.

For sun:
Brunnera macrophylla
Centaurea montana and C. montana 'Alba'
Achillea ptarmica
Veronica gentianoides

For sun or partial shade:
Astrantia
Alchemilla conjuncta
Bergenia x schmidtii
Polemonium
Geraniums (hardy)
Euphorbia polychroma
Leucanthemum x superbum
Pulmonaria
Primula 'Wanda'

I hope this is a help and that you can find something you like.

Thanks to Doctorbob 1 and Spritzhenry for their prompt replies to my query,very helpful.
The banks are very exposed,sun all day if ever we get a normal summer again,the plants you both suggested will be sorted as to colour etc.
Once again many thanks.
Billybob

Good luck with it - do let us know how you get on.