Yeah, frankly I think this says more about broadcast television than about Doctor Who. It was in fourth place for the day, with News at Ten, Casualty, and Blankety Blank above it - I think it's rather clear what the demographics are for broadcast TV.
I like this chart for comparing the current series' performance to past ones, because it relies on seeing it against the wider television landscape rather than just as abstract numbers. Doctor Who may be getting lower over-the-air ratings, but everything is - its been pretty impressively stable.
I'm more excited for this episode than I have been for any episode this series. Inua Ellams is awesome and a perfect fit for Doctor Who.