Salesmen are just for that purpose - force a deal through to get their commission no matter what. The issue here is that the DFS salesman is trying to swindle you by leveraging on your lack of knowledge about your own rights as a consumer.
If you were to walk into a car showroom let's say Arnold Clark and you agree to buy a car costing 18,000 pounds, sign credit application and all the numerous forms. At that point, the car is still NOT yours and you can't take it home(same as for your sofa) and the guy will say it'll be ready in a week's time(approx). The next morning the car salesman calls you up and says your application didn't go through. Let's say you have only 300 pounds in hand to spend, nothing in the bank. Does that mean you are obliged to pay 18,000 pounds for a BMW you chose to buy? Or does it force you to bring a relative of yours and get them into debt? The answer is a simple NO. If you don't get credit, you can't buy it and you just walk away no matter what any tom dick and harry salesman says. If he's already ordered it, he can cancel it or stock it that's his headache(if not the trick of the trade to tell you that).
Since you didn't get credit, just take a note of that company name to which you applied for credit and tell them that as your application was refused on so and so date(or rather first confirm if it was refused indeed), you're NOT buying any products and any claims made for credit by DFS relating to the application you made on xyz date should be treated as null and void.
Secondly, irrespective of whether you got that company details or not, don't honor the salesman's demands. It's not just you...we all can see how stupid this is : forcing a customer to pay up or get relatives into debt when your application for credit was clearly refused. So just don't give in to their tall tales unless you wish to fund the salesman's christmas prezzies! If he calls, tell him "as the credit was refused, I can't buy it and a NO means NO - there are no other options from my end no matter what you suggest" and don't pay any deposits or sign further documentation either.It's a closed chapter.
And thanks for informing us all about it.