Initially you have to write to the lender better by recorded, stating the reasons why you consider you were missold the PPI policy.. asking that premiums are returned and for them to restore the loan to the same state as it would be had the policy not been added. This option is open to anyone, it is always worth a try as they can either agree and make a refund, or simply refuse.
To be honest I cannot guess on the outcome, the fact you took it out in 2004 and have not challenged it by now might go against you,.... but as above you have nothing at all to lose and everything to gain by trying.!!
Hopefully, if the lenders do agree then they would refund this without further action on your behalf, however, if they refuse then after Eight weeks if you are still in dispute and consider the lenders response is unfair or unreasonable.....you then have the further choice to ask for the involvement of the financial ombudsman (below)...who have the power to investigate and overturn the lenders decision, if the FOS find in your favour.
http://www.financial-ombudsman.org.uk/faq/complain.html
Also, if I am unsuccessful could they demand I repay the outstanding loan and the credit card I have with them?
They are not supposed to take any punitive action against you just based solely on the fact you have made the above request, if they did, then that in itself would be deemed as unfair treatment by the FOS.