As a former licensee, and restaurant manager (or whatever you might call it, I waitressed and coordinated all the bookings in our small but busy restaurant), we used to only take deposits for christmas bookings, and sometimes for large bookings in our function room if we didnt know them. Some chose to pay for their bookings in full (it was a set christmas menu) before they came for their meal as they found this easier than a row on the night and also freed up their money for beer!! This was not the general rule though. We certainly didnt demand full payment. It was up to the customer if they decided to pay in full beforehand.
In short no, you shouldnt have been made to pay the full cost having already paid your deposit.
And I heard the salmon d'ella was also pretty good.......