My research keeps turning up Wood Asters (Eurybia species), but it still doesn't look quite right. I'll keep looking.

Aster (Eurybia) divaricatus. It may be a selected clone with particular characteristics not usually seen in wild populations.

It is indeed Aster divaricatus. The only named form I can find is Eastern Star.

It does look unusually lush, white, and large flowered, but it doesn't match with anything else I've found.

Thanks everyone, I've looked this up now and it does seem to match except for the odd ones that show it with green stems! One or two entries also say it flowers from June, but mine opened the last week of August. I'll try to remember to make a note next year.
I shall go tell the lady I bought it from-someone told her it was an Erigeron!!
Thanks again, I knew I could rely on all of you! The best plant encyclopaedia around!!