A nice surprise. Vinca difformis is what I thought I was planting, a relative of Greater and Lesser Periwinkle. The gardeners at the South London Botanical Institute gave me a few rooting stems last year. And sure enough the characteristic white flowers have appeared this spring.

 Unexpectedly there are some purple flowers among them which are another species of Periwinkle. A stem of Vinca major var. oxyloba must have got mixed up with the difformis. A happy accident- the combination of the two is delightful.