and insert this immediately after the words "...retaining these cartridges."
"The new printer was supplied by the manufacturer, but my contract was with you, and the faulty printer came complete with ink cartridges. I hold you responsible for the replacement of the missing cartridges, as at present the replacement printer is not fit for purpose.
I hereby reject the offer of £15 towards the purchase of new cartridges that was made to me by your customer services department on (insert date), and must insist that you honour your obligation under current consumer law without further delay. I reserve my rights in this matter."
Make sure you send the letter by special delivery - not by recorded delivery.