Mystery rose

This rose self-seeded into the gravel drive. I assumed it was a dog rose because they are common in the hedgrows round here, but now that it's flowering for the first time I can see that it is a much more complex form. The growth is strong and vigorous; the leaves are quite slender and bright green. The buds are cream, and the flowers break cream but then fade to white. Any ideas as to what it might be?