having problems with inaccurate p45 p60

blizzard1260

I work on a casual basis, every 3 months separated by 2 weeks leave so as not to get employment rights.When I come back, I am on BR code until I send employer's part of P45 to be back active on system. Last time I was on BR code for a number of weeks, when I was put on right code a significant amount of tax overpaid accumulated. My employer does everything in their power so that I don't get this money back. They are issueing false information on P45, P60 I have contacted them 7 times - it is going nowhere. They even updated HMRC with false income details for previous years and I had to go through struggle to sort this out otherwise I would be unlawfully paying back false tax underpayment. Is there a regulator in respect of correctness of P60 and P45? I understand HMRC are issueing instructions to employers in this respect.Can they act on my behalf?