According to Mark Jefferies:
BBC source has confirmed to me they will unveil new Doctor Who assistant tmrow. She is "pretty" but "not household name".
Does Sophia Myles count as "not a household name"? Given the Moff's urging that people should follow her on twitter, her recent travels to Spain and tweet from Arthur Darvill about meeting her it seems that some of the smart money is indeed on Sophia Myles. Or could this merely be a bit of extreme trolling?
We will find out tomorrow it seems.