Monarch tells Bigon that if it wasn't for Monarch, Bigon would still believe the Earth was flat. However, the Greeks knew the Earth was spherical (or nearly so, as it turns out), and had even calculated its circumference.
As the Doctor and Tegan enter the observation gallery, the bearded wrestler already seated there is wearing 1980s trainers; these have been removed by the next scene, where's he's now sitting down in the performance area.