Salesperson pushed an extended warranty that I now suspect is pointless

You may have paid for protection you already largely had. The Consumer Guarantees Act 1993 requires appliances to be of acceptable quality and durable for a reasonable time, and it applies regardless of any manufacturer's warranty. An extended warranty can add value, for example by covering situations that are not faults, but it cannot replace or reduce your statutory rights, and telling you otherwise can be misleading under the Fair Trading Act. Read the warranty terms and cancellation clause, then write to the seller asking for a refund on the basis that you were given misleading information about your existing rights. Keep any brochure or messages from the sale. If the seller declines, you can take the matter to the Disputes Tribunal or complain to the Commerce Commission.