We can trace the rumours back to Stephen Hart, the novelist. He posted to his Facebook page:
A friend of mine is playing in the orchestra at the Doctor Who proms. During the rehearsal last night they were told who the new Doctor is… and the big reveal will, I am told, be made officially at tonight’s Dr Who Proms first night. So who is it? I’m keeping my silence (I get enough &*&* about spoilers), but you *will* be very surprised…
This was later repurposed as an “exclusive” by one website, largely ignored by all others.
And that was a good call because tonight’s Doctor Who Prom is over now, and there was no new Doctor anywhere to be seen.
There were, however, old Doctors. And companions. And monsters. Some singers. And an Orchestra.