Ok, so I found the answer after reading something else on SO, which made me think a little further.
Initialising my subclassed IASKAppSettingsViewController with:
MyAppSettingsViewController *iaskvc = [[MyAppSettingsViewController alloc] initWithNibName:@"MyAppSettingsViewController" bundle:nil];
and adding my own xib file (which previously I didn't have) allowed me to customise everything I needed, and it then worked fine on the iPhone.