I may be a bit late with this reply.
I would take it to a garage that does 4 wheel alignment, most will give a print out of exactly how much it is out of alignment. Then armed with this information i would take it back to where you bought it from.
If there is no obvious visual damage they can't really blame you, if you had hit a kerb hard enough to move the subframe then you would expect wheel damage at least (check the spare wheel for damage, someone may of changed it)
As far as the warranty goes i think they have to cover the car for three months (regardless of the terms of their own warranty) just mention trading standards, vehicles have to be roadworthy
As for the mot depends how much its out really but would probably not be picked up, unless there is creased metal underneath the car.
You could do your own visual checks to the bonnet gaps (are they the same both sides and parallel) and the gap between the doors and wings etc. look for signs of paint overspray etc.
Also it might not be the actual subframe that is bent but the chassis leg it bolts to, look for any creases at the rear of the subframe where it bolts to the car