I'm in process of doing it myself (my letter sent to them and has been acknowledged). Mine is for a loan I took out in 2005 and they looking into it for me. Fingers crossed I'll get something back. Good luck with yours, they can only say No, worth giving it a go.
if you don't need it and want it on a credit card, you can take it off!
if it was added to a loan with equal payments you cannot remove it BUT you can apply to have whole lot repaid and have compo IF you can prove it was mis-sold!
I claimed PPI from a mortgage I took out in 2001 - 2005 (which I only had for 4 years with that particular lender) and my claim was successful, try it, see what happens..what have you got to lose.
It was PPI, I had the original paperwork - it was a separate payment added on to protect against sickness accident etc, I couldnt take the mortgage out without having this. So it was unfairly sold.