Yes, you do owe this. It is not the fault of the finance company that the laptop was not covered by insurance. When you buy something on finance, the finance company pay the retailer the full amount, then you pay the finance company.

First Credit are not the most reasonable people to deal with. I would suggest that you either borrow the money to get your discount, or offer to pay them in installments. Either way, you should put the matter to them in writing. They haven't got to the stage of issuing a CCJ, but there is nothing to stop them doing so should you ignore this.