Board Thread:The Panopticon/@comment-1827503-20140913002045/@comment-188432-20140918193537

Actually First Born has many references to Gwen being called "Mrs. Williams". It says that in the time between COE and MD, when living in that remote cottage, she did use "Williams" — to throw people off the track, so she would almost certainly have gone with "Williams" for Anwen, too. It's written in first person, alternating between Gwen and Rhys, so this is something the character directly states.

Here's the quote:
 * 'Yes?' I said. Just at the moment I was going about under Rhys's name. We'd figured the whole hiding- in-plain-sight thing would be easier that way. After all, you couldn't have an alarm go off every time a couple called Williams moved into a Welsh village, could you now?