Having seen the terms and conditions on their website click here
then I don't think you have to pay a deposit. However there is an administrative fee. have you paid that?
It doesn't say that the scheme applies to special orders. I can see why a company would want a deposit for special orders. If it is cancelled then they could be stuck with it.
Go back and talk politely to them explaining that a deposit for the 12 months interest free wasn't mentioned for special orders and is not in their website terms.