Complex sentence: Rix himself played the gormless north-country recruit, Horace Gregory, in both film and throughout the four-year run at the Whitehall, where his reputation for losing his trousers began.
Split into simpler sentences: Rix himself played the gormless north-country recruit, Horace Gregory, in both film and throughout the four-year run at the Whitehall. This is where his reputation for losing his trousers began.