Uplinker,
was not there a way to avoid returning to base?
In yr case there was discrepancy between IRS posn and GPS posn which resulted in corrupted FM posn but could not you correct the FM posn by overflying a navaid and then use "update at" function of the FMGC....
I am not sure if the above solution is correct.
How the Airbus plane navigates (FM, GPS, IRS...) Creates headache for me....