At a nuchal scan-which gave a "low risk" result-part of the membrane had come away from my uterus. Is this common?

Yes. Its not uncommon to see, on sono, something called a sub- chorionic hematoma. A small amount of bleeding has occurred under the membrane called the chorion and causes a separation. It usually resolves without problems. It has, occasionally been associated with abnormal early bleeding and the threat of a miscarriage. Usually everything is fine.
Yes. A separation between then chorion and your uterine wall is common before 16 weeks of pregnancy.