This has definitely become one of those questions I wish I hadn't asked!
I read an article which, typically, I can't find now which said City v Watford was the first Championship match Mark Heywood had taken charge of, but I've done some checking on Soccerbase. It turns out that there was a Mark Haywood and a Mark Heywood refereeing during the last decade and they were both from West Yorkshire;-
https://www.soccerbase.com/referees/...?tourn_id=1439
It was Mark Haywood who took charge of the Watford v City game in 2010 and Mark Heywood who did the City v Watford match in 2012, but there's a mistake in the question because it wasn't Heywood's first Championship match - it turns out he did another one in 11/12