Hi, if the details of your holidays are being stored somewhere, like for example documented on a system or files, then you are able to ask to see them under the Data Protection Act by making a ‘subject access request’ to the employer. They then have 40 days to respond to you with details or copies of the information you have requested.
In terms of proving whether you took the holidays or not that may not necessarily be easy but of course it would depend on the way this was done, whether there were any details in writing, or for example if you can use any witnesses which can verify that you did not take the time off etc. There is no specific way you can do this, it of curse depends on the circumstances and you will have to use whatever information is available out there to try and prove this is what happened.
The final option is to make a claim in the employment tribunal for the holidays owed and hopefully if it gets that far the employer will just pay you in order to avoid having to go through the claims process and defend the claim.
Hope this clarifies your position? If you could please let me know that would be great, thank you