I would guess your car was registered by dealer as they were not going to sell it new with the MY07 313 engine being around for some time before Sept 2007, especially when the 57 plate period arrived. IIRC the 313's were available early in the 07 plate registration period.
I hate to say this, but I think your car was sitting around for over 6 months unresgistered and either during that period or after the dealer registered it, it got damaged noting your previous posts on the rusting arch, and frankly a bodge job was done on that repair, adding to a somewhat dubious sale.
I do suggest you thoroughly check how the car was advertised and what you have in writing and if sufficiently armed go back to the supplying garage and demand to see the dealer principal as I am afraid I see the garage taking advantage of your inexperience......... Nissan are within their rights to decline 'interest' as I suspect that what I have said is what they have deduced as well.
You may wish to check out your position first with your local trading standards office.