Carminat sat nav is 1000miles off course!?

Renault laguna 2003 initiale.Last christmas I drove to Slovakia. On my return to the uk the sat-nav seemed to get lost somewhere in Austria after I drove on a new unmapped section of road. Having been back almost a month, whenever I now switch it on it just wanders around Austria or the Czech republic. Any ideas how to reset it wihtout visiting the dreaded main dealer? Already updated the disc but still the same.