I think after 5 or 6 months it's not unreasonable of PC World to refuse a refund or exchange the printer.
It seems normal practice in the PC business to seek help from the manufacturer rather than the retailer as a result of the complex nature of the equipment.
Under your set of circumstances I would have initially contacted Lexmark and if they failed to resolve the matter then approach PC World. I've had instances where the manufacturer has told me to return a product and show a copy of the manufacturers email as confirmation.
You should always remember the fault might be within your own PC rather than with the peripheral. If this is the case the manufacturer may help you resolve your problem.