it is illegal for a company to withold your holiday pay fullstop
they cannot really carry them forward but in the situation when you were off sick, you are still entitled to payment for holidays, companies try and say "holiday years finished sorry you cant have them"
then they just margin the money,
if you make a written request for it and ask them to explain in writing why they cannot give it to you i guarantee they will just pay you for it ensure the letter is addressed to one of the managers and sent recorded delivery state you want a reply within 5 working days
if they fail to reply contact ACAS 08457 47 47 47 these are the government funded company who deal with cases like this
possibly speak to them first
hope this helps