WDB HAS 'GROSS NAV ERROR' WHEN FLYING OVER ATLANTIC OCEAN.

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THE SCHEDULED FLT TIME WAS 8 HRS AND 58 MINS AND I WAS ANTICIPATING AT LEAST 1 MORE LEG ON THIS DAY. I HAD FLOWN AS A '2 MAN FLC' MORE THAN ONCE IN A 30 DAY PERIOD; THEREFORE I WAS SUBJECT TO FAR PART 121.525; WHICH LIMITS MY 'FLT DECK DUTY' TIME TO 8 HRS. AT ABOUT 3 HRS INTO THE FLT I TURNED THE FLT DECK OVER TO MY FO AND THE IRO AND RETIRED TO THE CREW BUNK FOR CREW REST. APPROX 1 HR AND 30 MINS LATER THE IRO WHO HAD OCCUPIED MY (THE L) SEAT WOKE ME TO ADVISE ME THAT THE ACFT HAD BEEN NAVIGATED TO AN IMPROPER POS. OUR CLRNC WAS TO N60 W60 AND WE HAD CROSSED N61 W60. THE ERROR WAS DISCOVERED WHEN GANDER ATC ASKED WHICH POS OUR CLRNC WAS TO. AT THIS TIME GANDER CLRED THE ACFT TO THE SUBSEQUENT POS AND ADVISED THAT THEY (GANDER ATC) WOULD BE FILING A 'GROSS NAVIGATIONAL ERROR RPT.' OUR COMPANY PROCS VERY CLRLY EXPLAIN HOW TO PROPERLY VERIFY ACFT POS DURING INS NAV. CHKING PRESENT POS AGAINST CLRNCS AND SUBSEQUENT POS BEFORE PROCEEDING; AND PLOTTING ALL POS AS WELL AS MID POINTS TO VERIFY PROPER COURSE. WHEN I QUERIED THE 2 CREW MEMBERS OF THESE PROCS THEY SAID THEY WERE FAMILIAR WITH THEM BUT HAD NOT COMPLIED WITH THEM. NO FURTHER ANOMALIES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THAT THE ERROR OCCURRED BECAUSE OF AN INCORRECTLY LOADED POS IN THE INS. THE ERROR WOULD HAVE BEEN CAUGHT AND DEV WOULD HAVE BEEN MUCH LESS IF THE FO AND IRO HAD FOLLOWED COMPANY REQUIREMENTS TO VERIFY POS AS STATED. RPTR HAD NO CHOICE BUT TO LEAVE THE FLT DECK TO COMPLY WITH FARS.