] ... her reputation for *propriety* that Lady Middleton is worried about, but her reputation for elegance. 8 or 9 couples, 2 violins and refreshments on the side-board (rather than something more extensive) is just too casual and small-scale for London, in Lady M's opinion. What if the people she wants to impress were to hear about it?!?

I think that Lady Middleton worries much more about how elegant she appears when the family is in town. True, she was not too anxious to have the Miss Steeles stay in her home in case they might not be fashionable enough for her, but she can put up with Sir John's impromptu gatherings at Barton and just reprimand him about it five or six times a day. However, in London, as you say, people she wants to impress might find out about it.