My wife gave her current employer their P45 from their previous job (which ended late in the 2009/10 tax year) so that they could get all her necessary personal details from it. The employer brought in the gross pay and tax as current year and as a result she is paying tax that she shouldn't be paying. They now say they can't do anything to rectify the situation and that she'll have to speak to the local tax office.
I believe that it is the employer's responsibility to correct the error in their records (in the same way as if they'd just pulled figures out of thin air for previous pay and tax or entered somebody else's) so that the cumulative tax coding will then fix the problem automatically. It also seems likely that a P46 will be necessary, though this may confuse things if HMRC have already accepted Part 3 without noticing it related to the wrong year.
Am I correct? Since the employer is convinced they can't do anything , references to show to them would be much appreciated. (assuming they are wrong)