Salary Arrears after leaving employment - Follow up

I previously posted a question about receiving salary arrears after leaving employment and how my ex employer should treat it.  The general consensus from the responses I got was that my ex employer should process the payment through their payroll using a 0T code.  I informed my ex employer of the way they should do it, but yesterday I received an email saying the payment will be made gross and I have to account for my own tax.  I am going to refer them HMRC guidance and legislation detailing what they should have done but if they don't back down what is my next course of action?