You make the error that the people you’re dealing with actually have authority to negotiate. they’ll get a memo at the beginning of the week or day saying what discounts they are allowed to give. Shopping on a bank holiday is probably your only school-boy error. They’re probably the most busy shopping days for sofa companies. If you went this saturday, or during the week you very well may have got a discount.
I’ve never been that successful in negotiating discounts – nothing significant anyway. I think the only way to do it is to be prepared to walk away if they say no. Having said that I do tend to decide what I want and go to the place that the internet says offers the best price, so they’re already probably bottoming out the price. Sofa’s are different though.