If you are in the UK you may qualify for legal advice where you get a lawyer and your employer has to pay for it.
A couple of years back I resigned from a job in the UK and although I was on good terms with the people I worked with the HR department (inevitably) did their best to try and screw me and having a good lawyer on my side made the process fairly painless as they actually did the negotiation with my former employer and I got 3 months salary and various other benefits.