I reckon if you speak to Storm that they may come to some agreement as long as you talk to them nicely.
They will have seen that it is unused and i'm sure it will come with some guarantee, does the guy who you bought it off have a receipt or any details of the date that he bought it.
Sure it should not affect your guarantee if it was new and unused when it was sold to you.
Wouldn't give up and get a milltek, but wouldn't go stage 2 to aleviate the problem. I had a cat fitted to my decat for £215 last month. but sure storm will have some answers for you first.