Sorry - I got the wrong end of the chamber. If you look behind Mhairi Black in this clip you'll see who I mean, but it's not always the same person. During PMQs there's usually a man sitting in the single seat, always with the white round-necked collar.
The Clerk of the House sits at the table in front of the Speaker, so it's not him! I've only noticed this mysterious character since the Scot Nats gained a role in PMQs and he/she is always there in the single seat behind the SNP leader. Can anyone enlighten us?