FirstPlus PPI

We submitted a PPI claim to Barclays regarding a loan we had through FirstPlus, after a couple of letters saying it is being investigated we have had a letter from CapitalOne saying they took over HFS loans who were the broker for our loan. The letter states that we ticked the box and were also advised over the phone that it was our choice to take the PPI so therefore it is not their fault, I do not remember whether we ticked the box or not but I would have explained about being paid if I was ill.
I work for the same company still and was entitled to 6 months full sick pay, but the PPI was pushed as a cash back policy after 5 years with no claim.
CapitalOne say they are not responsible as I ticked the box, and they say for the front loading of the insurance policy and I need to go back to Barclays regarding a Plevin claim.
Who do I need to go to first, CapitalOne regarding the selling a policy even though I was covered, or FirstPlus/Barclays for the front loading of the PPI premium.

Thanks