Were 'natch' and 'trimmensely' 50s slang or Marlows-only? I thought they might be the latter because they do appear in all the books.

If it was necessary to communicate, everybody used whatever was standard for their families.

That's what I'd have thought, and besides I can't imagine Patrick and the rather upper-class Marlows using it. Their own special words now, that would be different, like the odd nicknames used only at home. Gilly for Giles? Ha, he deserves that.