clematis confusion

I planted what I thought was a henrii and it has flowered three years running now but not until late september. I always believed henrii to flower may/june. The flower heads are white/single/showy about 6" across,
has anyone any ideas as to what might it actually be??
regards

 

Sounds as though it could be C. henryi.

Crocus http://www.crocus.co.uk/plants/_/climbers/clematis/large-flowering/classid.887/ says flowering June to August. And this site http://www.clematischoice.co.uk/shop/index.php?main_page=product_info&products_id=54 says there are only a few pure white Clematis - especially ones with huge flowers.