I think it IS Clifford Norgate, even though they're doing something weird with the effect on his voice. For the end of the same scene in Episode 9, Clifford Norgate is credited as Smirkenorff so that's probably not a direct lie. Maybe they did it specially to match the Bhal-Shebah "dragon" voice, having forgotten to try it out in Episode 7, then realised it was a rubbish idea and abandoned it for Episode 10.
They do put a similarly weird, slowing effect on Smirky's voice (though not nearly so extreme as this one) when he first speaks to Simon in S7E2, which I think provides a traceable bridge between his normal voice and his "Dunstan" voice. There is no sign of this effect after he actually takes off in that episode, however.