Right Skull, only found my notes (rough) but can't find my text books so here goes.
When buying food in a Restaurant you form a contract like any other contract, the 4 elements have to be met, offer, acceptance, consideration and intent to create legal relation.
In a restaurant, the menu is the "invitation to treat" but the offer is made when the waiter brings over the food, not when you ordered (this bit i'd never forget, remember a workshop on this bit very vividly). Acceptance comes usually when the waiter comes over to check is everything is okay with your food. Even if no one comes, an act can amount to implied acceptance (Brogden v Metropolitan Railway 1877 and also see G Percy Trentham v Archital Luxfer Ltd 1992).
So buy eating the food, you are actively accepting the terms of the contract, if it's not acceptable then you should reject the goods, where is this line? Like I said before, it's at the 50% mark.