Time for claim after made redundant

I was made redundant in 2015 after 6 years of continuous employment, I have since moved on but only recently realised my redundancy payment did not include my untaken holidays. I was not aware at the time that holidays were to be included in the redundancy payment and being a bit distressed during the process, I did not take the needed precautions against exploitation (I did not go seek for Unions support/advice),

Can I still claim the money I am owed, or there is a time limit and it is too late?

Can someone please help on this?