As a footnote to that Gatiss actor substitution: turns out it's Fisayo Akinade, a mate of Russell T Davies and I immediately suspect the hand of the Gatiss/Davies/Moffatt triumverate. I suppose we're lucky it didn't transform into a quasi-Dr Who episode halfway through as the BBC Dracula mini-series appears to be doing (I haven't yet seen the conclusion).
EDITED: 11 Jan 2020 12:42 by WILLIAMA