Repairs not carried out but paid for

A few months ago I brought my car to a reputable Dealer for repairs. I now found out by accident ,the some of the parts charged for were in fact not fitted. The garage has admitted this is the case but states it was not by design but error- what are my options ?
Thanks pimgin

Have them complete the work that was originally paid for or ask for a refund for the portion of the bill that covers what wasn't done. If you go for the refund option you may need to negotiate the number of hours labour that is to be refunded along with the obvious cost of the parts.