I've used a claim company to get mine back - with success, one bank loan PPI already refunded and the other one agreed but not yet actually got the money. They take 20% but to me it's worth it - they know the wording to use etc and do all the chasing.
You don't have to go with with a claim company, you can do it yourself, but I'd rather let someone else do it for me.
It doesn't matter that you haven't kept the paperwork - the ones I have claimed for are fairly recent but I am told that the loan/card companies will provide all copies of back information for a small fee.