We seem to have black bars on widescreen HD programmes for some reason, including Demon Headmaster:
(click for bigger version)
I know the Butterfly drama from a few months ago was similar but IIRC those were thinner than these.
Is this the start of a relentless march towards 2.35:1?
Oh and to stay on topic, the Headmaster is a marvellous man and this is the best school I've ever been to
It's 2:1: the same as Doctor Who, and the aspect ratio that Netflix and Amazon require their commissions to be in.