Which clematis is this? The small white flowers point to a particular group

This is a small-flowered white autumn clematis, most closely resembling sweet autumn clematis, Clematis terniflora, or a very similar type. The photograph shows simple white star-shaped flowers in small clusters and fairly rounded, pale green buds, which fit this group better than the familiar large-flowered clematis varieties.

Flowering late in the season, in September, also fits. These clematis tend to produce masses of smaller flowers rather than a few large ones. This photograph alone cannot reliably distinguish all the closely related small-flowered white varieties, but it clearly belongs to the autumn-flowering, small-flowered group.

To get closer to the species or variety, check whether it has a noticeable scent, how large the complete flower clusters become and what the leaves look like further along the stems. After flowering, compare the seedheads too: they often reveal more than a single flower.