Just reply and tell her that you and the mum's were wanting a more casual lunch - Trinity is very formal, and pricey and not appropriate for just before xmas..
Just very politely tell her that you were organising it, and that it was only ever meant to be a relatively informal event, and that perhaps she could organise a more formal get together next time..
If you all want to meet and let your hair down, Trinity is not exactly the place.. How about a nice gastro pub?
Good luck, but stand your ground.. I would call her and talk to her, as I don't think her behaviour is appropriate... You can be polite ;o)