Erroneous Payments and IRP5

I've been overpaid erroneously for a year . My employer wants the monies paid back in full. Roughly a third of the amount was taxed and appears on the IRP5. Must the IRP5 be amended by the employer, while we sort out the pay back arrangements?

Yes they need to fix the IRP5 as your taxes paid are probably too high and so everything should be put into a place it should be as if the error didnt happen.