If they are doing this, they may be doing it verbally which will be really hard to prove. This might help
"Do I have a right to see a reference written about me?
Your previous or current employer doesn't have to show you a reference they've written about you. So if you want to see a reference that's been written about you, you should make a request to the employer the reference was sent to. You should make this request under the Data Protection Act. The employer will then work out whether they can show you the reference, in line with the Data Protection Act. You can find out more about making a request to see a reference on the Information Commissioner's website at: www.ico.org.uk. Or you can call their helpline on 0303 123 1113. The helpline is open Monday to Friday, 9am to 5pm.
Can my employer write a bad reference about me?
Your employer has a duty to write an accurate reference about you, and shouldn't mislead the employer asking for the reference. But some information shouldn't be included in a reference, unless you agree to it. This is information like your medical records, or information about spent criminal convictions.
In England, Wales and Scotland, for more information about spent criminal convictions and work, see Do I have to tell my employers about my criminal record?
Your employer also mustn't give information in a reference or another document, for example, an e-mail, which is inaccurate, or which is deliberately wrong or misleading. If this has happened, you may be able to take action against your employer.
If you're in this situation, you should get advice from an experienced adviser, for example at a Citizens Advice Bureau. To search for details of your nearest CAB, including those that can give advice by email, click on nearest CAB."
link here
http://www.adviceguide.org.uk/england/work_e/work_rights_at_work_e/basic_rights_at_work.htm