If you've just had a letter from the CAA, then it's a paper exercise to complete the form, photocopy your medical, complete the payment form, and post it all off. It is just a case of renewing the actual paper licence.
I have just had to do this for my PPL(H), new licence turned up in the post about a week later. I thought they all had a 5 year expiry on them.
If it's not that, then some more details would help others explain what is going on.