I'd second Trocadero - was there for my mams 50th in July and had great atmosphere and food. It stays open very late aswell which is ideal for a large group who dont want to go to pub/club afterwards.
Don't know what your budget is but if it doesn't stretch to the Trocadero (which is brilliant) I'd suggest Luigi Malones, it has some great special offers for Christmas and has a function room downstairs for larger groups. It's always lively and staff are friendly and helpful. No connection with them just a satisfied customer.