PPI with DFS

It won't be 'hidden'. Though you may have chosen a separate policy.

But the way to find out is to ask them.

I've found paperwork from 2011 with the sales contract and finance contract but there seems to be no mention of PPI on either, only stain cover policy which we did indeed want. Is it possible the PPI element would have been hidden completely from the paperwork?

No chance whatsoever it can be hidden. It would be a breach of the consumer credit act for one reason and totally illogical for the other reason. To pay PPI you would have to borrow more. Your balance would be increased and a transaction would have to be there to do that.

Simalarly, we also bought another sofa from them in 2007 but sadly we have no paperwork left. I'm thinking of making a SAR, maybe they will still have the records, but what should I ask for to establish any evidence of PPI on our account?

A DSAR isnt focused. it is a request for data they hold on you. Assuming you repaid the money before the 2011 credit agreement, you would expect the records held to be minimal or even non-existent.

The PPI issue was already known by 2007 and that is the period that firms began to stop offering it. It is unusual for PPI to be added to interest free delayed repayment credit. If you did have it, it normally kicked in after the delayed repayment interest-free period. If you paid it off before repayments started, then you are unlikely to have paid PPI even if you agreed to have it.