Aircraft crew talking of 'dirty' Leeds - the irony!
For those non-British reading, Leeds is a northern working class industrial city which, at that time (before the claen air acts of the 20th century) was indeed 'mucky' and grimy because of the factories.
The posh Croft was a naive upper class boy, and they were gently teasing him, before Gresham admonishes them!
The irony is, McDowell was born in Leeds!