Well, my first thought is that you bought it in August and used it, so rejecting outright is not really an option. You do however statutory rights under SOGA 1979 and one of them is that the item you bought should be "as described". If you were sold second-hand goods as new (albeit ex-display) then the "not as described" applies. You can also rely on the "not of satisfactory quality", obviously.
Be that as it may, if the printer is faulty, they can not offer you vouchers, store policy is one thing and statutory rights another. However, you seem unclear in your own mind as to whether you want the printer fixed or refunded, so you need to decide which way to go.
The extended warranty is another matter. You need to clarify whether the warranty of which the guy on the phone spoke refers to the original 12 mths warranty (which I suspect is the case) or the extended one. If it is the original one, then it has no effect on the extended one and you have spent your money on a product you might now be regretting to have bought, but there's no recourse for regret, I'm afraid.

OTOH, if the extended warranty is null and void because the printer was second-hand (I don't see why, then again I don't work for them!), then you were mis-sold it in the first place and should go after them for a full refund on those grounds, that you were sold a product which couldn't cover you in the first place.
Finally, when you say "i am also wondering if the extended warranty was worth buying as well at a cost of £75", my answer would be "no". You have statutory rights and they already cover you for most of the stuff that the warranty does. But that's for future reference.

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Advice & opinions given by Bookworm are personal, are not endorsed by Consumer Action Group or Bank Action Group, and are offered informally, without prejudice & without liability. Your decisions and actions are your own, and should you be in any doubt, you are advised to seek the opinion of a qualified professional.